Definition: magnetic disk

A major computer storage device. Increasingly, magnetic disks are being replaced by solid state drives (see SSD). Like magnetic tape, the bits on 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. To review the many removable disk products manufactured in the past, see magnetic disk history. For more disk details, see hard disk and hard disk interfaces.

Tracks and Bits
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 in one orientation or the other (0 or 1), and the smaller the bit, the greater the storage. See areal density and perpendicular recording.

Sectors
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 determines how to fit the 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.

Fault Tolerant in 1992

This prototype of a RAID fault-tolerant disk system was built by University of Berkeley graduate students. Housing 36 320MB disk drives, its total storage was less than one disk drive in the cheapest PC only six years later. (Image courtesy of The Computer History Museum.) See RAID.