In a Personal Computer

8/8/2019 In a Personal Computer

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Hard Disk Drive MaintenanceRecommendationsAll hard disk drives have similar structure and every model of hard disk drive mayfail. The hard disk drive reliability depends mainly on external environment. Themain reasons for disks failure are:

1. Overheating.2. Impact effect on both hard disk drive and computer case.3. Boosted computer case vibration due to the CD-ROM work and fans with

slackness in the bearings.4. Substandard supply unit.5. Frequent power on/off of computer or use of power-saving mode of the hard

disk drive.

Power on your PC when you begin computing on any given day and don't power itoff until you finish for the day, unless you need to keep your PC on all the time.

Some people turn their PC on and off throughout the day. Big mistake. Thiscauses a computer's innards to expand and contract frequently, creating stressthat can lead to premature component failure.
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All hard disk drives have similar structure andevery model of hard disk may fail if they are not used properly.Some main reasons for disk failure are overheating, impacteffect on both hard disk drive and computer case, boostedcomputers case vibration due to the CD-ROM work and fans with

slackness in the bearings, substandard supply unit, and frequentpower on/off of the computer or use of power saving mode ofthe hard disk drive. Power on your PC when you begincomputing on any given day and dont power it off until youfinish for that particular day, unless you need to keep your PC onall the time. Do you know that people have big mistake everytime they turn their PC on and off throughout the day, becauseaccording to what I have read it causes a computers innards toexpand and contract and contract frequently, creating stressthat can lead to premature component failure.

Hard driveA computer's main storage media device, also called a hard diskdrive or abbreviated as HD or HDD. The hard drive was firstintroduced on September 13, 1956 and consists of one or more harddisk platters inside of air sealed casing. Most hard drives arepermanently stored in an internal drive bay at the front of thecomputer and are connected with either ATA, SCSI, or a SATA cableand power cable. Below is an illustration of the inside of a hard diskdrive.
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As can be seen in the above picture of the desktop hard drive the mainsix components that help make up the inside are the head actuator,read/write actuator arm, read/write head, spindle, and platter.

Additional technical support and help on hard disk drives can be

found on our hard disk drive help page.

Information about installing a IDE/EIDE hard disk drive can be

found on document CH000413.

Hard disks are one of the most important and also one of the most interestingcomponents within the PC. They have a long and interesting history dating backto the early 1950s. Perhaps one reason that I find them so fascinating is how wellengineers over the last few decades have done at improving them in everyrespect: reliability, capacity, speed, power usage, and more.
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This excellent chart shows the evolution of IBM hard disks over the past15 years. Severaldifferent form factors are illustrated, showing the progress that they have

made over theyears in terms of capacity, along with projections for the future. 250 GBhard disks inlaptops in five years? Based on past history, there's a good chance that itwill in fact happen!Note that the scale on the left is logarithmic, not linear, and PC hard diskshave one actuator.Image IBM CorporationImage used with permission.

Hard Disk Drives

The hard disk drive in your system is the "data center" of the PC. It is here that allof your programs and data are stored between the occasions that you use thecomputer. Your hard disk (or disks) are the most important of the various types ofpermanent storage used in PCs (the others being floppy disks and other storagemedia such as CD-ROMs, tapes, removable drives, etc.) The hard disk differs fromthe others primarily in three ways: size (usually larger), speed (usually faster) andpermanence (usually fixed in the PC and not removable).
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Hard disk drives are almost as amazing as microprocessors in terms of thetechnology they use and how much progress they have made in terms ofcapacity, speed, and price in the last 20 years. The first PC hard disks had acapacity of 10 megabytes and a cost of over $100 per MB. Modern hard diskshave capacities approaching 100 gigabytes and a cost of less than 1 cent per MB!This represents an improvement of 1,000,000% in just under 20 years, or around

67% cumulative improvement per year. At the same time, the speed of the harddisk and its interfaces have increased dramatically as well.

Top view of a 36 GB, 10,000 RPM, IBM SCSIserver hard disk, with its top cover removed.Note the height of the drive and the 10 stacked platters.(The IBM Ultrastar 36ZX.)Original image IBM CorporationImage used with permission.

Your hard disk plays a significant role in the following important aspects of your

computer system:

Performance: The hard disk plays a very important role in overall system

performance, probably more than most people recognize (though that ischanging now as hard drives get more of the attention they deserve). Thespeed at which the PC boots up and programs load is directly related tohard disk speed. The hard disk's performance is also critical whenmultitasking is being used or when processing large amounts of data suchas graphics work, editing sound and video, or working with databases.
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Storage Capacity: This is kind of obvious, but a bigger hard disk lets youstore more programs and data.

Software Support: Newer software needs more space and faster harddisks to load it efficiently. It's easy to remember when 1 GB was a lot of diskspace; heck, it's even easy to remember when 100 MB was a lot of diskspace! Now a PC with even 1 GB is considered by many to be "crippled",

since it can barely hold modern (inflated) operating system files and acomplement of standard business software.

Reliability: One way to assess the importance of an item of hardware is toconsider how much grief is caused if it fails. By this standard, the hard diskis the most important component by a long shot. As I often say, hardwarecan be replaced, but data cannot. A good quality hard disk, combined withsmart maintenance and backup habits, can help ensure that the nightmareof data loss doesn't become part of your life.

This chapter takes a very detailed look at hard disks and how they work. Thisincludes a full dissection of the internal components in the drive, a look at how

data is formatted and stored, a discussion of performance issues, and a fullanalysis of the two main interfaces used to connect hard disks to the rest of thePC. A discussion is also included about the many confusing issues regarding harddisks and BIOS versions, and support for the newer and larger hard disks currentlyon the market. Finally, a full description is given of logical hard disk structures andthe functioning of the FAT and NTFS file systems, by far the most popularcurrently used by PCs.

Construction and Operation of the Hard Disk

To many people, a hard disk is a "black box" of sorts--it is thought of as just asmall device that "somehow" stores data. There is nothing wrong with thisapproach of course, as long as all you care about is that it stores data. If you useyour hard disk as more than just a place to "keep stuff", then you want to knowmore about your hard disk. It is hard to really understand the factors that affectperformance, reliability and interfacing without knowing how the drive worksinternally. Fortunately, most hard disks are basically the same on the inside.While the technology evolves, many of the basics are unchanged from the first PChard disks in the early 1980s.


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Photograph of a modern SCSI hard disk, with major componentsannotated.

The logic board is underneath the unit and not visible from this angle.Original image Western Digital CorporationImage used with permission.

In this section we dive into the guts of the hard disk and discover what makes ittick. We look at the various key components, discuss how the hard disk is puttogether, and explore the various important technologies and how they worktogether to let you read and write data to the hard disk. My goal is to go beyondthe basics, and help you really understand the design decisions and tradeoffsmade by hard disk engineers, and the ways that new technologies are beingemployed to increase capacity and improve performance.

Construction and Operation of the Hard Disk

To many people, a hard disk is a "black box" of sorts--it is thought of as just asmall device that "somehow" stores data. There is nothing wrong with thisapproach of course, as long as all you care about is that it stores data. If you useyour hard disk as more than just a place to "keep stuff", then you want to knowmore about your hard disk. It is hard to really understand the factors that affectperformance, reliability and interfacing without knowing how the drive worksinternally. Fortunately, most hard disks are basically the same on the inside.
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While the technology evolves, many of the basics are unchanged from the first PChard disks in the early 1980s.

Photograph of a modern SCSI hard disk, with major componentsannotated.The logic board is underneath the unit and not visible from this angle.Original image Western Digital CorporationImage used with permission.

In this section we dive into the guts of the hard disk and discover what makes ittick. We look at the various key components, discuss how the hard disk is puttogether, and explore the various important technologies and how they worktogether to let you read and write data to the hard disk. My goal is to go beyondthe basics, and help you really understand the design decisions and tradeoffs

made by hard disk engineers, and the ways that new technologies are beingemployed to increase capacity and improve performance.

Hard Disk Operational Overview

As an illustration, I'll describe here in words how the various components in thedisk interoperate when they receive a request for data. Hopefully this will providesome context for the descriptions of the components that follow in later sections.
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A hard disk uses round, flat disks called platters, coated on both sides with aspecial media material designed to store information in the form of magneticpatterns. The platters are mounted by cutting a hole in the center and stackingthem onto a spindle. The platters rotate at high speed, driven by a special spindlemotorconnected to the spindle. Special electromagnetic read/write devices calledheads are mounted onto sliders and used to either record information onto the

disk or read information from it. The sliders are mounted onto arms, all of whichare mechanically connected into a single assembly and positioned over thesurface of the disk by a device called an actuator. A logic board controls theactivity of the other components and communicates with the rest of the PC.

Each surface of each platter on the disk can hold tens of billions of individual bitsof data. These are organized into larger "chunks" for convenience, and to allow foreasier and faster access to information. Each platter has two heads, one on thetop of the platter and one on the bottom, so a hard disk with three platters(normally) has six surfaces and six total heads. Each platter has its informationrecorded in concentric circles called tracks. Each track is further broken down into

smaller pieces called sectors, each of which holds 512 bytes of information.

The entire hard disk must be manufactured to a high degree of precision due tothe extreme miniaturization of the components, and the importance of the harddisk's role in the PC. The main part of the disk is isolated from outside air toensure that no contaminants get onto the platters, which could cause damage tothe read/write heads.


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Exploded line drawing of a modern hard disk, showing the major components Though the specifics vary greatly between different designs, the bascomponents you see above are typical of almost all PC hard disks.Original image Seagate Technology

Image used with permission.

Here's an example case showing in brief what happens in the disk each time apiece of information needs to be read from it. This is a highly simplified examplebecause it ignores factors such as disk caching, error correction, and many of theother special techniques that systems use today to increase performance andreliability. For example, sectors are not read individually on most PCs; they aregrouped together into continuous chunks called clusters. A typical job, such asloading a file into a spreadsheet program, can involve thousands or even millionsof individual disk accesses, and loading a 20 MB file 512 bytes at a time would berather inefficient:

1. The first step in accessing the disk is to figure out where on the disk to lookfor the needed information. Between them, the application, operatingsystem, system BIOS and possibly any special driver software for the disk,do the job of determining what part of the disk to read.

2. The location on the disk undergoes one or more translation steps until afinal request can be made to the drive with an address expressed in termsof its geometry. The geometry of the drive is normally expressed in terms ofthe cylinder, head and sector that the system wants the drive to read. (Acylinder is equivalent to a track for addressing purposes). A request is sent
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to the drive over the disk drive interface giving it this address and asking forthe sector to be read.

3. The hard disk's control program first checks to see if the informationrequested is already in the hard disk's own internal buffer (or cache). It if isthen the controller supplies the information immediately, without needing tolook on the surface of the disk itself.

4.In most cases the disk drive is already spinning. If it isn't (because powermanagement has instructed the disk to "spin down" to save energy) thenthe drive's controller board will activate the spindle motor to "spin up" thedrive to operating speed.

5. The controller board interprets the address it received for the read, andperforms any necessary additional translation steps that take into accountthe particular characteristics of the drive. The hard disk's logic programthen looks at the final number of the cylinder requested. The cylindernumber tells the disk which track to look at on the surface of the disk. Theboard instructs the actuator to move the read/write heads to theappropriate track.

6. When the heads are in the correct position, the controller activates the headspecified in the correct read location. The head begins reading the tracklooking for the sector that was asked for. It waits for the disk to rotate thecorrect sector number under itself, and then reads the contents of thesector.

7. The controller board coordinates the flow of information from the hard diskinto a temporary storage area (buffer). It then sends the information overthe hard disk interface, usually to the system memory, satisfying thesystem's request for data.

Hard Disk Platters and Media

Every hard disk contains one or more flat disks that are used to actually hold thedata in the drive. These disks are called platters (sometimes also "disks" or"discs"). They are composed of two main substances: a substrate material thatforms the bulk of the platter and gives it structure and rigidity, and a magneticmedia coating which actually holds the magnetic impulses that represent thedata. Hard disks get their name from the rigidity of the platters used, ascompared to floppy disks and other media which use flexible "platters" (actually,they aren't usually even called platters when the material is flexible.)

The platters are "where the action is"--this is where the data itself is recorded. Forthis reason the quality of the platters and particularly, their media coating, iscritical. The surfaces of each platter are precision machined and treated toremove any imperfections, and the hard disk itself is assembled in a clean roomto reduce the chances of any dirt or contamination getting onto the platters.

Platter Size


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The size of the platters in the hard disk is the primary determinant of its overallphysical dimensions, also generally called the drive's form factor; most drives areproduced in one of the various standard hard disk form factors. Disks aresometimes referred to by a size specification; for example, someone will talkabout having a "3.5-inch hard disk". When this terminology is used it usuallyrefers to the disk's form factor, and normally, the form factor is named based on

the platter size. The platter size of the disk is usually the same for all drives of agiven form factor, though not always, especially with the newest drives, as we willsee below. Every platter in any specific hard disk has the same diameter.

The first PCs used hard disks that had a nominal size of 5.25". Today, by far themost common hard disk platter size in the PC world is 3.5". Actually, the plattersof a 5.25" drive are 5.12" in diameter, and those of a 3.5" drive are 3.74"; buthabits are habits and the "approximate" names are what are commonly used. Youwill also notice that these numbers correspond to the common sizes for floppydisks because they were designed to be mounted into the same drive bays in thecase. Laptop drives are usually smaller, due to laptop manufacturers' never-

ending quest for "lighter and smaller". The platters on these drives are usually2.5" in diameter or less; 2.5" is the standard form factor, but drives with 1.8" andeven 1.0" platters are becoming more common in mobile equipment.

A platter from a 5.25" hard disk, with a platterfrom a 3.5" hard disk placed on top of it forcomparison. The quarter is included for scale (andstrangely, fits right in the spindle hole for bothplatters... isn't that a strange coincidence?Traditionally, drives extend the platters to as much of the width of the physicaldrive package as possible, to maximize the amount of storage they can pack intothe drive. However, as discussed in the section on hard disk historical trends, thetrend overall is towards smallerplatters. This might seem counter-intuitive; afterall, larger platters mean there is more room to store data, so shouldn't it be more
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cost-effective for manufacturers to make platters as big as possible? There areseveral reasons why platters are shrinking, and they are primarily related toperformance. The areal density of disks is increasing so quickly that the loss ofcapacity by going to smaller platters is viewed as not much of an issue--fewpeople care when drives are doubling in size every year anyway!--whileperformance improvements continue to be at the top of nearly everyone's wish

list. In fact, several hard disk manufacturers who were continuing to produce5.25" drives for the "value segment" of the market as recently as 1999 have nowdiscontinued them. (The very first hard disks were 24" in diameter, so you can seehow far we have come in 40 or so years.)

Here are the main reasons why companies are going to smaller platters even fordesktop units:

Enhanced Rigidity: The rigidityof a platter refers to how stiff it is. Stiffplatters are more resistant to shock and vibration, and are better-suited forbeing mated with higher-speed spindles and other high-performance

hardware. Reducing the hard disk platter's diameter by a factor of twoapproximately quadruples its rigidity.

Manufacturing Ease: The flatness and uniformity of a platter is critical toits quality; an ideal platter is perfectly flat and consistent. Imperfect platterslead to low manufacturing yield and the potential for data loss due to theheads contacting uneven spots on the surface of a platter. Smaller plattersare easier to make than larger ones.

Mass Reduction: For performance reasons, hard disk spindles areincreasing in speed. Smaller platters are easier to spin and require less-powerful motors. They are also faster to spin up to speed from a stoppedposition.

Power Conservation: The amount of power used by PCs is becoming moreand more of a concern, especially for portable computing but even on thedesktop. Smaller drives generally use less power than larger ones.

Noise and Heat Reduction: These benefits follow directly from theimprovements enumerated above.

Improved Seek Performance: Reducing the size of the platters reducesthe distance that the head actuator must move the heads side-to-side toperform random seeks; this improves seek time and makes random readsand writes faster. Of course, this is done at the cost of capacity; you couldtheoretically achieve the same performance improvement on a larger disk

by only filling the inner cylinders of each platter. In fact, some demandingcustomers used to partition hard disks and use only a small portion of thedisk, for exactly this reason: so that seeks would be faster. Using a smallerplatter size is more efficient, simpler and less wasteful than this sort of"hack".

The trend towards smaller platter sizes in modern desktop and server drivesbegan in earnest when some manufacturers "trimmed" the platters in their10,000 RPM hard disk drives from 3.74" to 3" (while keeping them as standard
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3.5" form factor drives on the outside for compatibility.) Seagate's Cheetah X1515,000 RPM drive goes even further, dropping the platter size down to 2.5", againtrading performance for capacity (it is "only" 18 GB, less than half the size ofmodern 3.5" platter-size drives.) This drive, despite having 2.5" platters, still usesthe common 3.5" form factor for external mounting (to maintain compatibilitywith standard cases), muddying the "size" waters to some extent (it's a "3.5-inch

drive" but it doesn't have 3.5" platters.)

The smallest hard disk platter size available on the market today is a miniscule 1"in diameter! IBM's amazing Microdrive has a single platter and is designed to fitinto digital cameras, personal organizers, and other small equipment. The tinysize of the platters enables the Microdrive to run off battery power, spin down andback up again in less than a second, and withstand shock that would destroy anormal hard disk. The downside? It's "only" 340 MB. :^)

Internal view and dimensions of the amazing IBM Microdrive.Image IBM CorporationImage used with permission.

Here's a summary table showing the most common platter sizes used in PCs, inorder of decreasing size (which in most cases is also chronological order fromtheir data of introduction, but not always) and also showing the most commonform factors used by each technology:

PlatterDiameter

Typical Form Factor Application

5.12 5.25" Oldest PCs, used in servers through the mid-1990s and some retail drives in the mid-to-late1990s; now obsolete

3.74 3.5"Standard platter size for the most common harddisk drives used in PCs

3.0 3.5" High-end 10,000 RPM drives

2.5 2.5", 3.5"Laptop drives (2.5" form factor); 15,000 RPMdrives (3.5" form factor)

1.8 PC Card (PCMCIA) PC Card (PCMCIA) drives for laptops

1.3 PC Card (PCMCIA)Originally used on hand-held PCs (no longermade)
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1.0 CompactFlashDigital cameras, hand-held PCs and otherconsumer electronic devices

Number of Platters

Hard disks can have one platter, or more, depending on the design. Standard

consumer hard disks, the type probably in your PC right now, usually havebetween one and five platters in them. Some high-end drives--usually used inservers--have as many as a dozen platters. Some very old drives had even more.In every drive, all the platters are physically connected together on a commoncentral spindle, to form a single assembly that spins as one unit, driven by thespindle motor. The platters are kept apart using spacer rings that fit over thespindle. The entire assembly is secured from the top using a cap or cover andseveral screws. (See the spindle motor page for an illustration of thesecomponents.)

Each platter has two surfaces that are capable of holding data; each surface has aread/write head. Normally both surfaces of each platter are used, but that is notalways the case. Some older drives that use dedicated servo positioning reserveone surface for holding servo information. Newer drives don't need to spend asurface on servo information, but sometimes leave a surface unused formarketing reasons--to create a drive of a particular capacity in a family of drives.With modern drives packing huge amounts of data on a single platter, using onlyone surface of a platter allows for increased "granularity". For example, IBM'sDeskstar 40GV family sports an impressive 20 GB per platter data capacity. SinceIBM wanted to make a 30 version of this drive, they used three surfaces (on twoplatters) for that drive. Here's a good illustration of how Western Digital createdfive different capacities using three platters in their Caviar line of hard disk drives:

Model Number

NominalSize(GB)

Data Sectors PerDrive

Platters Surfaces

WD64AA 6.4 12,594,960 1 2

WD102AA 10.2 20,044,080 2 3

WD136AA 13.6 26,564,832 2 4

WD172AA 17.2 33,687,360 3 5

WD205AA 20.5 40,079,088 3 6

Note: In theory, using only one surface means manufacturing costs can be savedby making use of platters that have unacceptable defects on one surface, but Idon't know if this optimizing is done in practice...
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From an engineering standpoint there are several factors that are related to thenumber of platters used in the disk. Drives with many platters are more difficult toengineer due to the increased mass of the spindle unit, the need to perfectly alignall the drives, and the greater difficulty in keeping noise and vibration undercontrol. More platters also means more mass, and therefore slower response tocommands to start or stop the drive; this can be compensated for with a stronger

spindle motor, but that leads to other tradeoffs. In fact, the trend recently hasbeen towards drives with fewerhead arms and platters, not more. Areal densitycontinues to increase, allowing the creation of large drives without using a lot ofplatters. This enables manufacturers to reduce platter count to improve seek timewithout creating drives too small for the marketplace. See here for more on thistrend.

This Barracuda hard disk has 10 platters.(I find the choice of fish hooks as abackground for this shot highly amusing. :^) )

Original image Seagate Technology Image used with permission.

The form factor of the hard disk also has a great influence on the number ofplatters in a drive. Even if hard disk engineers wanted to put lots of platters in aparticular model, the standard PC "slimline" hard disk form factor is limited to 1inch in height, which limits the number of platters that can be put in a single unit.Larger 1.6-inch "half height" drives are often found in servers and usually havemany more platters than desktop PC drives. Of course, engineers are constantlyworking to reduce the amount of clearance required between platters, so they canincrease the number of platters in drives of a given height.

Platter Substrate Materials

The magnetic patterns that comprise your data are recorded in a very thin medialayer on the surfaces of the hard disk's platters; the bulk of the material of theplatter is called the substrate and does nothing but support the media layer. Tobe suitable, a substrate material must be rigid, easy to work with, lightweight,stable, magnetically inert, inexpensive and readily available. The most commonlyused material for making platters has traditionally been an aluminum alloy, whichmeets all of these criteria.
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Due to the way the platters spin with the read/write heads floating just abovethem, the platters must be extremely smooth and flat. With older, slower spindledrives and relatively high fly heights, the uniformity of the platter surface was lessof an issue. Now, as technology advances, the gap between the heads and theplatter is decreasing, and the speed that the platters spin at is increasing,creating more demands on the platter material itself. Uneven platter surfaces on

hard disks running at faster speeds with heads closer to the surface are more aptto lead to head crashes. For this reason many drive makers began several yearsago to look at alternatives to aluminum, such as glass, glass composites, andmagnesium alloys.

Hard disk platters are very smooth, right? Well, not to a scanningelectron microscope!The image on the left is of the surface of an aluminum alloy platter; the

one on the rightis a glass platter. The images speak for themselves. The scale is inmicrons..Composed from two original images IBM CorporationImages used with permission.

It now is looking increasingly likely that glass and composites made with glass willbe the next standard for the platter substrate. IBM has been shipping drives withglass platters for several years and in 2000 is introducing them into the IDE/ATAconsumer drive market. Compared to aluminum platters, glass platters haveseveral advantages:

Better Quality: The first and most important reason for going to glass isprobably that glass platters can be made much smoother and flatter thanaluminum, improving the reliability of the hard disk and making low flyingheights and faster spindle speeds more feasible.

Improved Rigidity: Another important consideration is that glass is more

rigid than aluminum for the same weight of material. Improved rigidity, oneof the reasons why platter sizes are also shrinking in size, is important forreducing noise and vibration with drives that spin at high speed.
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Thinner Platters: The enhanced rigidity of glass also allows platters to bemade thinner than with aluminum, allowing more platters to be packed intothe same drive dimensions. Thinner platters also weigh less, reducingspindle motor requirements and reducing start time when the drive is atrest.

Thermal Stability: When heated, glass expands much less than does

aluminum. With some hard disk platters now containing 35,000 tracks perinch or more, even a small amount of expansion can causes these tracks to"move around". The drive's servo mechanism compensates for expansionand contraction, but it is still preferable to use materials that move as littleas possible because this reduces the amount of adjusting the hard drive hasto do, improving performance.

One obvious disadvantage of glass compared to aluminum is fragility, particularlywhen made very thin. For this reason some companies are experimenting withglass/ceramic composites. One of these is a Dow Corning product called MemCor,which is a glass made with ceramic inserts to reduce the likelihood of cracking.

Sometimes these composites are just called "glass", much the way aluminumalloy platters, which usually contain other metals, are just called "aluminum".

Magnetic Media

The substrate material of which the platters are made forms the base upon whichthe actual recording media is deposited. The media layer is a very thin coating ofmagnetic material which is where the actual data is stored; it is typically only afew millionths of an inch in thickness.

Older hard disks used oxide media. "Oxide" really means iron oxide--rust. Of

course no high-tech company wants to say they use rustin their products, so theyinstead say something like "high-performance oxide media layer". :^) But in factthat's basically what oxide media is, particles of rust attached to the surface ofthe platter substrate using a binding agent. You can actually see this if you look atthe surface of an older hard disk platter: it has the characteristic light brown color.This type of media is similar to what is used in audio cassette tape (which has asimilar color.)

Oxide media is inexpensive to use, but also has several important shortcomings.The first is that it is a soft material, and easily damaged from contact by aread/write head. The second is that it is only useful for relatively low-density

storage. It worked fine for older hard disks with relatively low data density, but asmanufacturers sought to pack more and more data into the same space, oxidewas not up to the task: the oxide particles became too large for the smallmagnetic fields of newer designs.

Today's hard disks use thin film media. As the name suggests, thin film mediaconsists of a very thin layer of magnetic material applied to the surface of theplatters. (While oxide media certainly isn't thick by any reasonable use of theword, it was much thicker than this new media material; hence the name "thin
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film".) Special manufacturing techniques are employed to deposit the mediamaterial on the platters. One method is electroplating, which deposits thematerial on the platters using a process similar to that used in electroplatingjewelry. Another is sputtering, which uses a vapor-deposition process borrowedfrom the manufacture of semiconductors to deposit an extremely thin layer ofmagnetic material on the surface. Sputtered platters have the advantage of a

more uniform and flat surface than plating. Due to the increased need for highquality on newer drives, sputtering is the primary method used on new diskdrives, despite its higher cost.

Compared to oxide media, thin film media is much more uniform and smooth. Italso has greatly superior magnetic properties, allowing it to hold much more datain the same amount of space. Finally, it's a much harder and more durablematerial than oxide, and therefore much less susceptible to damage.

A thin film 5.25" platter (above) next to an oxide 5.25" platter (below).Thin film platters are actually reflective; taking photographs of themis like trying to take a picture of a mirror! This is one reason whycompanies always display internal hard disk pictures at an angle.After applying the magnetic media, the surface of each platter is usually coveredwith a thin, protective, layer made of carbon. On top of this is added a super-thinlubricating layer. These material are used to protect the disk from damage causedby accidental contact from the heads or other foreign matter that might get intothe drive.

IBM's researchers are now working on a fascinating, experimental new substancethat may replace thin film media in the years ahead. Rather than sputtering ametallic film onto the surface, a chemical solution containing organic moleculesand particles of iron and platinum is applied to the platters. The solution is spreadout and heated. When this is done, the iron and platinum particles arrangethemselves naturally into a grid of crystals, with each crystal able to hold amagnetic charge. IBM is calling this structure a "nanocrystal superlattice". Thistechnology has the potential to increase the areal density capability of the
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recording media of hard disks by as much as 10 or even 100 times! Of course it isyears away, and will need to be matched by advances in other areas of the harddisk (particularly read/write head capabilities) but it is still pretty amazing andshows that magnetic storage still has a long way to go before it runs out of roomfor improvement.

Magnetic Media

The substrate material of which the platters are made forms the base upon whichthe actual recording media is deposited. The media layer is a very thin coating ofmagnetic material which is where the actual data is stored; it is typically only afew millionths of an inch in thickness.

Older hard disks used oxide media. "Oxide" really means iron oxide--rust. Ofcourse no high-tech company wants to say they use rustin their products, so theyinstead say something like "high-performance oxide media layer". :^) But in factthat's basically what oxide media is, particles of rust attached to the surface of

the platter substrate using a binding agent. You can actually see this if you look atthe surface of an older hard disk platter: it has the characteristic light brown color.This type of media is similar to what is used in audio cassette tape (which has asimilar color.)

Oxide media is inexpensive to use, but also has several important shortcomings.The first is that it is a soft material, and easily damaged from contact by aread/write head. The second is that it is only useful for relatively low-densitystorage. It worked fine for older hard disks with relatively low data density, but asmanufacturers sought to pack more and more data into the same space, oxidewas not up to the task: the oxide particles became too large for the small

magnetic fields of newer designs.

Today's hard disks use thin film media. As the name suggests, thin film mediaconsists of a very thin layer of magnetic material applied to the surface of theplatters. (While oxide media certainly isn't thick by any reasonable use of theword, it was much thicker than this new media material; hence the name "thinfilm".) Special manufacturing techniques are employed to deposit the mediamaterial on the platters. One method is electroplating, which deposits thematerial on the platters using a process similar to that used in electroplatingjewelry. Another is sputtering, which uses a vapor-deposition process borrowedfrom the manufacture of semiconductors to deposit an extremely thin layer of

magnetic material on the surface. Sputtered platters have the advantage of amore uniform and flat surface than plating. Due to the increased need for highquality on newer drives, sputtering is the primary method used on new diskdrives, despite its higher cost.

Compared to oxide media, thin film media is much more uniform and smooth. Italso has greatly superior magnetic properties, allowing it to hold much more datain the same amount of space. Finally, it's a much harder and more durablematerial than oxide, and therefore much less susceptible to damage.
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A thin film 5.25" platter (above) next to an oxide 5.25" platter (below).Thin film platters are actually reflective; taking photographs of them

is like trying to take a picture of a mirror! This is one reason whycompanies always display internal hard disk pictures at an angle.After applying the magnetic media, the surface of each platter is usually coveredwith a thin, protective, layer made of carbon. On top of this is added a super-thinlubricating layer. These material are used to protect the disk from damage causedby accidental contact from the heads or other foreign matter that might get intothe drive.

IBM's researchers are now working on a fascinating, experimental new substancethat may replace thin film media in the years ahead. Rather than sputtering ametallic film onto the surface, a chemical solution containing organic molecules

and particles of iron and platinum is applied to the platters. The solution is spreadout and heated. When this is done, the iron and platinum particles arrangethemselves naturally into a grid of crystals, with each crystal able to hold amagnetic charge. IBM is calling this structure a "nanocrystal superlattice". Thistechnology has the potential to increase the areal density capability of therecording media of hard disks by as much as 10 or even 100 times! Of course it isyears away, and will need to be matched by advances in other areas of the harddisk (particularly read/write head capabilities) but it is still pretty amazing andshows that magnetic storage still has a long way to go before it runs out of roomfor improvement.

Tracks and Sectors

Platters are organized into specific structures to enable the organized storage andretrieval of data. Each platter is broken into tracks--tens of thousands of them--which are tightly-packed concentric circles. These are similar in structure to theannual rings of a tree (but not similar to the grooves in a vinyl record album,which form a connected spiral and not concentric rings).
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A track holds too much information to be suitable as the smallest unit of storageon a disk, so each one is further broken down into sectors. A sector is normallythe smallest individually-addressable unit of information stored on a hard disk,and normally holds 512 bytes of information. The first PC hard disks typically held17 sectors per track. Today's hard disks can have thousands of sectors in a singletrack, and make use ofzoned recording to allow more sectors on the larger outer

tracks of the disk.

A platter from a 5.25" hard disk, with 20 concentric tracks drawnover the surface. This is far lower than the density of even the oldesthard disks; even if visible, the tracks on a modern hard disk wouldrequire high magnification to resolve. Each track is divided into16 imaginary sectors. Older hard disks had the same number ofsectors per track, but new ones use zoned recording with a differentnumber of sectors per track in different zones of tracks.A detailed examination of tracks and sectors leads into a larger discussion of diskgeometry, encoding methods, formatting and other topics.

Areal Density

Areal density, also sometimes called bit density, refers to the amount of data thatcan be stored in a given amount of hard disk platter "real estate". Since diskplatters surfaces are of course two-dimensional, areal density is a measure of thenumber of bits that can be stored in a unit of area. It is usually expressed in bitsper square inch (BPSI).
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Being a two-dimensional measure, areal density is computed as the product oftwo other one-dimensional density measures:

Track Density: This is a measure of how tightly the concentric tracks onthe disk are packed: how many tracks can be placed down in inch of radiuson the platters. For example, if we have a platter that is 3.74" in diameter,

that's about 1.87 inches. Of course the inner portion of the platter is wherethe spindle is, and the very outside of the platter can't be used either. Let'ssay about 1.2 inches of length along the radius is usable for storage. If inthat amount of space the hard disk has 22,000 tracks, the track density ofthe drive would be approximately 18,333 tracks per inch (TPI).

Linear or Recording Density: This is a measure of how tightly the bits arepacked within a length of track. If in a given inch of a track we can record200,000 bits of information, then the linear density for that track is 200,000bits per inch per track (BPI). Every track on the surface of a platter is adifferent length (because they are concentric circles), and not every track iswritten with the same density. Manufacturers usually quote the maximum

linear density used on each drive.

Taking the product of these two values yields the drive's areal density, measuredin bits per square inch. If the maximum linear density of the drive above is300,000 bits per inch of track, its maximum areal density would be 5,500,000,000bits per square inch, or in more convenient notation, 5.5 Gbits/in 2.The newestdrives have areal densities exceeding 10 Gbits/in2, and in the lab IBM in 1999reached 35.3 Gbits/in2--524,000 BPI linear density, and 67,300 TPI track density!In contrast, the first PC hard disk had an areal density of about 0.004 Gbits/in2!

Note: Sometimes you will see areal density expressed in a different way:gigabytes per platter (GB/platter). This unit is often used when comparing drives,and is really a different way of saying the same thing--as long as you are alwaysclear about what the platter size is. It's easier conceptually for many people tocontrast two units by saying, for example: "Drive A has 10 GB/platter and drive Bhas 6 GB/platter, so A has higher density". Also, it's generally easier to computewhen the true areal density numbers are not easy to find. As long as they bothhave the same platter size, the comparison is valid. Otherwise you are comparingapples and oranges.

The linear density of a disk is not constant over its entire surface--bear this inmind when reading density specifications, which usually list only the maximumdensity of the disk. The reason that density varies is because the lengths of thetracks increase as you move from the inside tracks to the outside tracks, so outertracks can hold more data than inner ones. This would mean that if you stored thesame amount of data on the outer tracks as the inner ones, the linear density ofthe outer tracks would be much lower than the linear density of the inner tracks.
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This is in fact how drives used to be, until the creation of zoned bit recording,which packs more data onto the outer tracks to exploit their length. However, theinner tracks still generally have higher density than the outer ones, with densitygradually decreasing from the inner tracks to the outer ones.

Tip: Here's how you can figure this out yourself. A typical 3.5" hard disk drive hasan innermost track circumference of about 4.75" and an outermost trackcircumference of about 11". The ratio of these two numbers is about 2.32. Whatthis means is that there is 2.32 times as much "room" on the outer tracks. If youlook at the number of sectors per track on the outside zone of the disk, and it isless than 2.32 times the number of sectors per track of the inside zone, then youknow the outer tracks have lower density than the inner ones. Consider the IBM40GV drive, whose zones are shown in a table on this page on zoned bitrecording. Its outermost tracks have 792 sectors; its innermost tracks 370. This is

a ratio of 2.14, so assuming this drive uses tracks with the lengths I mentioned, ithas its highest density on the innermost tracks.

There are two ways to increase areal density: increase the linear density bypacking the bits on each track closer together so that each track holds more data;or increase the track density so that each platter holds more tracks. Typically newgeneration drives improve both measures. It's important to realize that increasingareal density leads to drives that are not just bigger, but also faster, all else beingequal. The reason is that the areal density of the disk impacts both of the keyhard disk performance factors: both positioning speed and data transfer rate. See

this section for a full discussion of the impact of areal density on hard diskperformance.
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This illustration shows how areal density works. First, divide thecircle in your mind into a left and right half, and then a top and bottomhalf.The left half shows low track density and the right half high trackdensity.The upper half shows low linear density, and the bottom half high

linear density.Combining them, the upper left quadrant has the lowest areal density;theupper right and lower left have greater density, and the bottom rightquadrant of course has the highest density.(I hope you like this illustration, because you don't want to knowwhat I went through trying to make it... :^) )Increasing the areal density of disks is a difficult task that requires manytechnological advances and changes to various components of the hard disk. Asthe data is packed closer and closer together, problems result with interferencebetween bits. This is often dealt with by reducing the strength of the magnetic

signals stored on the disk, but then this creates other problems such as ensuringthat the signals are stable on the disk and that the read/write heads are sensitiveand close enough to the surface to pick them up. In some cases the heads mustbe made to fly closer to the disk, which causes other engineering challenges, suchas ensuring that the disks are flat enough to reduce the chance of a head crash.Changes to the media layer on the platters, actuators, control electronics andother components are made to continually improve areal density. This isespecially true of the read/write heads. Every few years a read/write headtechnology breakthrough enables a significant jump in density, which is why hard


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disks have been doubling in size so frequently--in some cases it takes less than ayear for the leading drives on the market to double in size.

Hard Disk Read/Write Heads

The read/write heads of the hard disk are the interface between the magnetic

physical media on which the data is stored and the electronic components thatmake up the rest of the hard disk (and the PC). The heads do the work ofconverting bits to magnetic pulses and storing them on the platters, and thenreversing the process when the data needs to be read back.

Read/write heads are an extremely critical component in determining the overallperformance of the hard disk, since they play such an important role in thestorage and retrieval of data. They are usually one of the more expensive parts ofthe hard disk, and to enable areal densities and disk spin speeds to increase, theyhave had to evolve from rather humble, clumsy beginnings to being extremelyadvanced and complicated technology. New head technologies are often the

triggering point to increasing the speed and size of modern hard disks.

hard Disk Read/Write Head Operation

In many ways, the read/write heads are the most sophisticated part of the harddisk, which is itself a technological marvel. They don't get very much attention,perhaps in part because most people never see them. This section takes a look athow the heads work and discusses some of their key operating features andissues.

In a Personal Computer

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