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A disk drive is a peripheral device that uses rotational motion to seek, read, and write data. The drive contains a motor to spin the disk at a constant or variable rate and one or more read/write heads which are positioned over the desired track by a servo mechanism. It also contains the electronics to amplify the signals from the heads to normal digital logic levels and vice versa.[1] Popular configurations for data storage have included floppy disks, hard disk drives, and optical drives. However, all these mechanical devices are being phased out in favor of faster solid-state drives.[2]


In order for a disk drive to start to read or write a given location a read/write head must be positioned radially over the correct track and rotationally over the start of the corresponding sector. The radial motion is referred to as "seeking", which causes most of the intermittent noise heard during disk activity. There is usually one head for each disk surface and all heads move together. The set of locations which are accessible with the heads in a given radial position are known as a "cylinder". The "seek time" is the time taken to seek to a different cylinder.

The disk normally remains in constant rotation (except for floppy or portable hard disk drives where the motor may be switched off after a set period of inactivity to reduce wear and power consumption). Positioning the heads over the right sector is simply a matter of waiting until it arrives under the head. With a single set of heads this "rotational latency" will be on average half a revolution but some big drives have multiple sets of heads spaced at equal angles around the disk.

If seeking and rotation are independent, access time is seek time + rotational latency. When accessing multiple tracks sequentially, data is sometimes arranged so that by the time the seek from one track to the next has finished, the disk has rotated just enough to begin accessing the next track.[1] When large blocks of data cannot be written to a single contiguous space on the disk, this may trigger additional seek times from reading back the data from multiple locations on the physical surface of the media, known as fragmentation.[3]


IBM announced the first magnetic disk drive on September 14, 1956 as part of its 305 RAMAC data processing system.[4] Early drives had a capacity of only a few megabytes, and were housed inside a separate cabinet the size of a washing machine. Over a few decades they shrunk to fit a terabyte or more in a box the size of a paperback book.[1]

Some disk configurations may be removable, such as floppy disks and optical drives. Large removable hard disks were common on mainframes and minicomputers. Smaller form factors, such as SyQuest and Jaz drives, were introduced in the 1980s and 1990s for use by personal computers in desktop publishing, but these mostly fell out of use by the early 2000s.[5]


  1. 1.0 1.1 1.2 Disk drive at the Free On-Line Dictionary Of Computing. 1997-04-15.
  2. Solid State Disk at the Free On-Line Dictionary Of Computing. 2013-04-27.
  3. How to safely defrag a Mac's hard drive by ds store, Apple Communities. 2013-06-16.
  4. 650 RAMAC announcement, IBM Archives. 1956-09-14.
  5. Iomega Jaz Drive, Digital Scrapbooking Storage. Accessed 2021-08-31.

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FOLDOC logo This page uses GFDL licensed content from the Free On-line Dictionary of Computing.