Secondary storage devices are auxiliary storage devices that are used to store data and programs when they are not being processed. Secondary storage is more permanent than the main memory, as data and programs are retained even when the power is turned off. The need for secondary storage can vary greatly between users. A personal computer might only require 20 Mega bytes of secondary storage but large companies may require secondary storage devices that can store billions of characters. Because of such a variety of needs, a variety of storage devices are available in the market. Some of the secondary storage devices are discussed as under:
(i) Magnetic tapes
Magnetic tape is a one-half inch or one-quarter inch ribbon of plastic material on which data is recorded. The tape drive is an input/output device that reads, writes and erases data on tapes. Magnetic tapes are erasable, reusable and durable. They are made to store large quantities of data inexpensively and therefore are often used for backup. Magnetic tape is not suitable for data files that are revised or updated often because it stores data sequentially.
(ii) Magnetic disks
Magnetic disks are the most widely used storage medium for computers. A magnetic disk offers high storage capacity, reliability, and the capacity to directly access stored data. Magnetic disks hold more data in a small place and attain faster data access speeds. Types of magnetic disks include diskettes, hard disks, and removable disk cartridges.
(a) Diskettes: The diskette was introduced in the early 1970s by IBM as a new type of secondary storage. Originally they were eight inches in diameter and were thin and flexible which gave them the name floppy disks, or floppies. Diskettes are used as the principle medium of secondary storage for personal computers. They are available in two different sizes: 3 1/2 inch and 5 1/4 inch.
(b) Hard disks: Hard disks provide larger and faster secondary storage capabilities than diskettes. Usually hard disks are permanently mounted inside the computer and are not removable like diskettes. On minicomputers and mainframes, hard disks are often called fixed disks. They are also called direct-access storage devices (DASD).
(c) Disk Cartridges: Removable disk cartridges are another form of disk storage for personal computers. They offer the storage and fast access of hard disks and the portability of diskettes. They are often used when security is an issue since, when a person has finished using the computer, the disk cartridge can be removed and locked up leaving no data on the computer.
(d) Removable-Pack Disk Systems: It consists of hard disks stacked into a pack or an individual unit that can be mounted or removed as a unit. They are typically found on mainframe and minicomputer systems. A typical disk pack has 11 disks, each with two surfaces. Only 20 surfaces on the disk can be used for recording data, the top and the bottom surfaces are not used. Each surface area is divided into tracks, where the data is recorded.
(e) Winchester Disk Systems: These disks are hermetically sealed units that cannot be removed from the disk drive. They are typically used in microcomputers and have capacities in the range of 20—30 GB.
(f) Zip Disks: These are high-capacity floppy disk drives developed by Iomega Corporation. They are slightly larger than the conventional floppy disks, and are about twice as thick. They can hold 750 MB of data.
(g) Jaz Disk: These are removable disk drives. It has a 12-ms average seek time and a transfer rate of 5.5 Mbps. They can hold 1 GB of data.
(h) REV Drive: REV drives are the latest secondary storage devices launched by Iomega Corporation. It provides removable storage with hard disk performance and can store upto 90 GB of compressed data.
(i) USB Drives: USB drives use the USB port on the computer for data transfer. Mini Drives launched by Iomega Corporation can hold upto 1 GB data and are extremely small in size. The dimension of a mini drive is (2.22 cm Width, 7.30 cm Length, 1.11 cm Height)
(j) Optical Disks: Optical disk is a disk in which light is the medium used to record and read data. The disk is made of clear polycarbonate plastic, covered with a layer of dye, a thin layer of gold, which reflects the laser beam, and a protective layer over that. A recording is made by sending pulses from a laser beam, which make a pattern in the layer of dye. The recording is read later by directing a laser beam at the disk and interpreting the pattern of reflected light. CDs, CD-ROMs, and video discs, are commercially recorded optical disks and are not rewritable.
Recordable optical disks include WORM (write once read many) disks, and CD-Rs (CD-recordables), which can be written only once; and CD-Es (CD-erasables), which can be rewritten many times. A Digital Video Disk or DVD has a much larger capacity than a CD, even though both are the same size, physically. They both read and write data in similar ways, and all recordable DVD drives can also record CDs. But, a single DVD disc has the capability to store up to 13 times the amount of data contained on a CD - on one side alone. Since both sides of a DVD can be used for data storage that means DVDs can offer up to 26 times the storage of a Compact Disc
Currently, there are five different recordable formats: DVD-R, DVD-RW, DVD-RAM, DVD+R, and DVD+RW The following chart shows the capacities/sizes of various recordable media:
CD-R and CD-RW - 0.65GB to 0.7GB
DVD-R - 3.95GB or 4.7GB
DVD-RW - 4.7GB
DVD+R - 4.7GB
DVD+RW - 4.7GB
DVD-RAM - 2.6GB to 9GB
(i) Magnetic tapes
Magnetic tape is a one-half inch or one-quarter inch ribbon of plastic material on which data is recorded. The tape drive is an input/output device that reads, writes and erases data on tapes. Magnetic tapes are erasable, reusable and durable. They are made to store large quantities of data inexpensively and therefore are often used for backup. Magnetic tape is not suitable for data files that are revised or updated often because it stores data sequentially.
(ii) Magnetic disks
Magnetic disks are the most widely used storage medium for computers. A magnetic disk offers high storage capacity, reliability, and the capacity to directly access stored data. Magnetic disks hold more data in a small place and attain faster data access speeds. Types of magnetic disks include diskettes, hard disks, and removable disk cartridges.
(a) Diskettes: The diskette was introduced in the early 1970s by IBM as a new type of secondary storage. Originally they were eight inches in diameter and were thin and flexible which gave them the name floppy disks, or floppies. Diskettes are used as the principle medium of secondary storage for personal computers. They are available in two different sizes: 3 1/2 inch and 5 1/4 inch.
(b) Hard disks: Hard disks provide larger and faster secondary storage capabilities than diskettes. Usually hard disks are permanently mounted inside the computer and are not removable like diskettes. On minicomputers and mainframes, hard disks are often called fixed disks. They are also called direct-access storage devices (DASD).
(c) Disk Cartridges: Removable disk cartridges are another form of disk storage for personal computers. They offer the storage and fast access of hard disks and the portability of diskettes. They are often used when security is an issue since, when a person has finished using the computer, the disk cartridge can be removed and locked up leaving no data on the computer.
(d) Removable-Pack Disk Systems: It consists of hard disks stacked into a pack or an individual unit that can be mounted or removed as a unit. They are typically found on mainframe and minicomputer systems. A typical disk pack has 11 disks, each with two surfaces. Only 20 surfaces on the disk can be used for recording data, the top and the bottom surfaces are not used. Each surface area is divided into tracks, where the data is recorded.
(e) Winchester Disk Systems: These disks are hermetically sealed units that cannot be removed from the disk drive. They are typically used in microcomputers and have capacities in the range of 20—30 GB.
(f) Zip Disks: These are high-capacity floppy disk drives developed by Iomega Corporation. They are slightly larger than the conventional floppy disks, and are about twice as thick. They can hold 750 MB of data.
(g) Jaz Disk: These are removable disk drives. It has a 12-ms average seek time and a transfer rate of 5.5 Mbps. They can hold 1 GB of data.
(h) REV Drive: REV drives are the latest secondary storage devices launched by Iomega Corporation. It provides removable storage with hard disk performance and can store upto 90 GB of compressed data.
(i) USB Drives: USB drives use the USB port on the computer for data transfer. Mini Drives launched by Iomega Corporation can hold upto 1 GB data and are extremely small in size. The dimension of a mini drive is (2.22 cm Width, 7.30 cm Length, 1.11 cm Height)
(j) Optical Disks: Optical disk is a disk in which light is the medium used to record and read data. The disk is made of clear polycarbonate plastic, covered with a layer of dye, a thin layer of gold, which reflects the laser beam, and a protective layer over that. A recording is made by sending pulses from a laser beam, which make a pattern in the layer of dye. The recording is read later by directing a laser beam at the disk and interpreting the pattern of reflected light. CDs, CD-ROMs, and video discs, are commercially recorded optical disks and are not rewritable.
Recordable optical disks include WORM (write once read many) disks, and CD-Rs (CD-recordables), which can be written only once; and CD-Es (CD-erasables), which can be rewritten many times. A Digital Video Disk or DVD has a much larger capacity than a CD, even though both are the same size, physically. They both read and write data in similar ways, and all recordable DVD drives can also record CDs. But, a single DVD disc has the capability to store up to 13 times the amount of data contained on a CD - on one side alone. Since both sides of a DVD can be used for data storage that means DVDs can offer up to 26 times the storage of a Compact Disc
Currently, there are five different recordable formats: DVD-R, DVD-RW, DVD-RAM, DVD+R, and DVD+RW The following chart shows the capacities/sizes of various recordable media:
CD-R and CD-RW - 0.65GB to 0.7GB
DVD-R - 3.95GB or 4.7GB
DVD-RW - 4.7GB
DVD+R - 4.7GB
DVD+RW - 4.7GB
DVD-RAM - 2.6GB to 9GB
