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Definition: magnetic disk

The primary computer storage device. Increasingly magnetic disks are being replaced by SSDs (see solid state drive). Like magnetic tape, disks are magnetically recorded and can be re-recorded over and over. Disks contain rotating platters with a mechanical arm that moves a read/write head across their surface. For more details, see hard disk and hard disk interfaces.

Tracks and Spots
A disk's surface is divided into concentric tracks (circles within circles), and the thinner the tracks, the more storage. The data bits are recorded as magnetic spots on the tracks, and the smaller the spot, the greater the storage. See areal density.

Tracks are further divided into sectors, which hold a block of data that is read or written at one time; for example, READ SECTOR 782, WRITE SECTOR 5448. In order to update the disk, one or more sectors are read into the computer, changed and written back to disk. The operating system figures out how to fit data into these fixed spaces.

Modern disks have more sectors in the outer tracks than the inner ones because the outer radius of the platter is greater than the inner radius (see CAV). See magnetic tape and optical disc.

Tracks and Sectors
Tracks are concentric circles that are broken up into storage units called "sectors," typically 4,096 bytes long. The sector is the smallest unit that can be read or written. Tracks are only 75 nanometers wide today, and bit density is greater than one terabit per square inch. See areal density.

Magnetic Disk Summary
The following magnetic disk technologies are summarized below. Except for the hard disk, all of the following drives are no longer manufactured. However, media such as floppy disks, Zip and Rev cartridges are often available.

The Early 1990s
This RAID II prototype in 1992, which embodies principles of high performance and fault tolerance, was designed and built by University of Berkeley graduate students. Housing 36 320MB disk drives, its total storage was less than the disk drive in the cheapest PC only six years later. (Image courtesy of The Computer History Museum, www.computerhistory.org) See RAID.