SECONDARY STORAGE DEVICES Secondary storage devices are also known as Auxiliary Storage Devices. The auxiliary storage devices store system programs, large data files, assemblers, compilers and other programs. In other words auxiliary storage devices are used for bulk storage of data. The storage capacity of these devices is unlimited as an empty device can replace them once the existing device is completely filled.

The secondary storage is permanent in nature i.e. the information stored in these devices does not lost unless it is specifically deleted. Secondary storage devices being permanent in nature can be used for the transportation of data from one computer to another.

Secondary storage devices are cheaper as compared to the main memory. The information stored in secondary memory are first transferred to the main memory and then processed by the CPU. The final result may then be placed in the secondary memory. It is because of this that the access time of the secondary memory is comparatively high.

There are four types of secondary storage devices which are discussed below. These are;

Hard Disk. Compact Disc. USB (Universal Significant Bit). Blu-Ray Disc.

HARD DISK DRIVEThe hard disk drive was primarily developed with the reason to provide a secondary storage device which is fast and have higher capacity and compactness as well as that is much reliable as a data storage system. As it is known that hard disk drive is basically an internal. In some cases it can also be external. The advantage regarding the external hard disk drive is that it is easy to move the data and user information from one place to another. The other forms of data transfer can be made on network itself. But the external hard disk drive is used to transfer the data that is quiet large enough; and which cannot be moved over the network.

The internal architecture of hard disk consists of four components; Chassis. Disk platter. Head arm. Head Actuator.

The chassis is the part of the hard disk which acts as a base over which other parts of the hard disk are placed. The disk platter is usually a magnetic material where all the data or information is stored over. The disk platter which has the data over it can be accessed with the help of a head arm that moves around the platter to access the data or the information. Head arm is controlled by another major component of the hard disk that is the Head Actuator.

The Head Actuator is an electronic device that is controlled by a rotating motor. It helps the Head arm in moving all over the hard disk platter. Usually in the earlier hard disk drives the Head Actuator that is present inside the hard disk drive was controlled by a stepper motor. Presently the newer hard disk drives have the commonly used motor as the servo motor that is used by the Head Actuator.

Hard disk drives have a large capacity to store large amount of data. Today highest capacity consumer HDDs are 3TB. HDDs have very high speed. The fastest enterprise HDDs spin at 10,000 or 15,000 rpm, and can achieve sequential media transfer speeds above 1.6 Gbit/s and a sustained transfer rate up to 1 Gbit/s. Drives running at 10,000 or 15,000 rpm use smaller platters to migrate increased power requirements and therefore generally have lower capacity than the highest capacity desktop drives.

CD-ROMA Compact Disc (also known as a CD) is an optical disc used to store digital data. It was originally developed to store sound recordings exclusively, but later it also allowed the preservation of other types of data. Standard CDs have a diameter of 120 mm and can hold up to 80 minutes of uncompressed audio(700 MB of data). The Mini CD has various diameters ranging from 60 to 80 mm; they are sometimes used for CD singles or device drivers, storing up to 24 minutes of audio. A CD is made from 1.2 mm thick, almost-pure polycarbonate plastic and weighs 15–20 grams. From the center outward, components are: the center (spindle) hole, the first-transition area (clamping ring), the clamping area (stacking ring), the second-transition area (mirror band), the information (data) area, and the rim. CD data are stored as a series of tiny indentations known as "pits", encoded in a spiral track molded into the top of the polycarbonate layer. The areas between pits are known as "lands". Each pit is approximately 100 nm deep by 500 nm wide, and varies from 850 nm to 3.5 µm in length.CD-ROM Data StorageAlthough the disc media and the drives of the CD and CD-ROM are, in principle, the same, there is a difference in the way data storage is organized. Two new sectors were defined, Mode 1 for storing computer data and Mode 2 for compressed audio or video/graphic data. CD-ROM Mode 1

CD-ROM Mode 1 is the mode used for CD-ROMs that carry data and applications only. In order to access the thousands of data files that may be present on this type of CD, precise addressing is necessary. data is stored in sectors (the smallest separately addressable block of information), which each hold 2,352 bytes of data, with an additional number of bytes used for error detection and correction, as well as control structures .There are 75 sectors per second on the disk, which yields a disc capacity of 681,984,000 bytes (650MB) and a single speed transfer rate of 150 Kbps, with higher rates for faster CD-ROM drives. Drive speed is expressed as multiples of the single speed transfer rate, as 2X, 4X, 6X, and so on. CD-ROM Mode 2CD-ROM Mode 2 is used for compressed audio/video information and uses only two layers of error detection and correction, the same as the CD-DA. Therefore, all 2,336 bytes of data behind the sync and header bytes are for user data. Although the sectors of CD-DA, CD-ROM Mode 1 and Mode 2 are the same size, the amount of data that can be stored varies considerably because of the use of sync and header bytes, error correction and detection. It allows different kinds of data to be mixed together, and became the basis for CD-ROM XA.

USB DEVICEIn the present time of advanced storage devices USB is another important storage device.USB stands for ‘UNIVERSAL SERIAL BUS’. A USB drive -- also known as a flash drive or keychain drive -- is a plug-and-play portable storage device that uses flash memory and is lightweight enough to attach to a key chain. A USB drive can be used in place of a floppy disk, Zip drive disk, or CD. When the user plugs the device into the USB port, the computer's operating system recognizes the device as a removable drive and assigns it a drive letter.

Unlike most removable drives, a USB drive does not require rebooting after it's attached, does not require batteries or an external power supply,. Several manufacturers offer additional features such as password protection, and downloadable drivers that allow the device to be compatible with older systems that do not have USB ports. USB drives are available in capacities ranging up to about 65 gigabytes (GB), depending on manufacturer, in a corresponding range of prices.

With a USB drive, data can be retained for long periods when the device is unplugged from the computer, or when the computer is powered-down with the drive left in. This makes the USB drive convenient for transferring data between a desktop computer and a notebook computer, or for personal backup needs.

Blu-ray DiscBlu-ray Disc is an optical disc storage medium designed to supersede the standard DVD format. Its main uses are for storing high-definition video, PlayStation 3 video games, and other data, with up to 25 GB per single-layered, and 50 GB per dual-layered disc. Although these numbers represent the standard storage for Blu-ray Disc drives, the specification is open-ended, with the upper theoretical storage limit left unclear. The discs have the same physical dimensions as standard DVDs and CDs.The name Blu-ray Disc derives from the "blue laser" used to read the disc. While a standard DVD uses a 650 nanometer red laser, Blu-ray Disc uses a shorter wavelength 405 nm laser, and allows for over five

times more data storage on single-layer and over ten times on double-layer Blu-ray Disc than a standard DVD. The laser color is called "blue," but is violet to the eye, and is very close to ultraviolet ("black light").

Blu-Ray Discs can be clustered together in systems such as optical jukeboxes to increase data storage. This increase of storage can span multiple terabytes and utilize hundreds of Blu-Ray Discs. These systems are currently the largest storage units using Blu-Ray technology.Technical specifications:Type Physical size Single layer capacity Dual layer capacity

Standard disc size 12 cm 25 GB /23866 MiB /25025314816 B 50 GB / 47732 MiB / 50050629632 B

Mini disc size 8 cm 7.8 GB / 7430 MiB / 7791181824 B 15.6 GB / 14860 MiB / 15582363648 B

Blu-ray Disc uses a "blue" laser, operating at a wavelength of 405 nm, to read and write data. The diodes are GaN (gallium nitride) lasers that produce 405 nm photons directly, that is, without frequency doubling or other nonlinear optical mechanisms. Conventional DVDs and CDs use red and near-infrared lasers, at 650 nm and 780 nm, respectively.

The blue-violet laser's shorter wavelength makes it possible to store more information on a 12 cm CD/DVD-size disc. The minimum "spot size" on which a laser can be focused is limited by diffraction, and depends on the wavelength of the light and the numerical aperture of the lens used to focus it. By decreasing the wavelength, increasing the numerical aperture from 0.60 to 0.85, and making the cover layer thinner to avoid unwanted optical effects, the laser beam can be focused to a smaller spot. This allows more information to be stored in the same area. For Blu-ray Disc, the spot size is 580 nm. In addition to the optical improvements, Blu-ray Discs feature improvements in data encoding that further increase the capacity.

Since the Blu-ray Disc data layer is closer to the surface of the disc compared to the DVD standard, it was at first more vulnerable to scratches. The first discs were housed in cartridges for protection, resembling Professional Discs introduced by Sony in 2003.

Using a cartridge would increase the price of an already expensive medium, so hard-coating of the pickup surface was chosen instead. TDK was the first company to develop a working scratch-protection coating for Blu-ray Discs. It was named Durabis. In addition, both Sony and Panasonic's replication methods include proprietary hard-coat technologies. Sony's rewritable media are spin-coated, using a scratch-resistant and antistatic coating. Verbatim's recordable and rewritable Blu-ray Discs use their own proprietary hard-coat technology, called Scratch Guard.

Blu-ray Disc specification allows the use of such a layer to meet the required scratch resistance. DVD media are not required to be scratch-resistant, but since development of the technology, some companies, such as Verbatim, implemented hard-coating for more expensive lineups of recordable DVDs.