SeConDaRy StoRaGe

Secondary storage or external memory differs from primary storage in that it is not directly accessible by the CPU. The computer usually uses its Input/output channels to access secondary storage and transfers the desired data using intermediate area in primary storage. Secondary storage does not lose the data when the device is powered down—it is non-volatile. Per unit, it is typically also an order of magnitude less expensive than primary storage. Consequently, modern computer systems typically have an order of magnitude more secondary storage than primary storage and data is kept for a longer time there.



• FLOPPY DISK

Floppy disk is a data storage medium that is composed of a disk of thin, flexible ("floppy") magnetic storage medium encased in a square or rectangular plastic shell.
Floppy disks are read and written by a floppy disk drive or FDD, the initials of which should not be confused with "fixed disk drive", which is another term for a (nonremovable) type of hard disk drive.



• FLOPPY DRIVES

A floppy drive grabs a disk at its centre and spin it insides its plastics jacket. The floppy drive obtains stored data and instruction from a floppy disk and stores them onto the disk. The drive is made up of a box with a slot into which a user inserts a disk. The slot has a drive gate. This drive rotates the disk with a motor inside the drive. Electronic read or write heads read data from the disk and write data to it while the disk rotates. A microcomputer usually has internal floppy drives inside the computer cabinet, but it sometimes has external floppy drive, a separate component outside the cabinet.




• HARD DISK

A hard disk drive (often shortened as hard disk, hard drive, or HDD) is a non-volatile storage device that stores digitally encoded data on rapidly rotating rigid (i.e. hard) platters with magnetic surfaces. Strictly speaking, "drive" refers to the motorized mechanical aspect that is distinct from its medium, such as a tape drive and its tape, or a floppy disk drive and its floppy disk. Early HDDs had removable media; however, an HDD today is typically a sealed unit (except for a filtered vent hole to equalize air pressure) with fixed media.



• OPTICAL DISKS

Optical disk is a any of a variety of information storage disks that are played or read using a laser. Optical disks include compact discs (CDs and CD-ROMs), laser discs and digital versatile discs (or digital video discs; DVDs and DVD-ROMs). WORM [Write Once/Read Many] disks can be used to record data, but once data is recorded it cannot be altered except by obliterating the old version and storing the new version on a previously unused portion of the disk. Magneto-optical disks, such as the rewritable optical disk and the recordable disk used with the Mini Disc player, have a special layer, as of barium ferrite, that can be magnetically polarized by a recording head when heated with a laser. Data or sound may be recorded to and erased from any portion of a magneto-optical disk multiple times.



• MAGNETIC TAPE

Magnetic tape is sequential storage medium used for data collection, backup and archiving. Like videotape, computer tape is made of flexible plastic with one side coated with a ferromagnetic material. Tapes were originally open reels, but were superseded by cartridges and cassettes of many sizes and shapes.
Tape has been more economical than disks for archival data, but that is changing as disk capacities have increased enormously. If tapes are stored for the duration, they must be periodically recopied or the tightly coiled magnetic surfaces may contaminate each other.



• CACHE (CACHE MEMORY)

To store data locally in order to speed up subsequent retrievals. Pronounced "cash." See Web cache and cache. Reserved areas of memory in every computer that are used to speed up instruction execution, data retrieval and data updating. Pronounced "cash," they serve as staging areas, and their contents are constantly changing. There are two kinds: memory caches and disk caches.