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Last Updated June 23, 2021 7:07 am

A disk (or diskette) is a flat, round plate on which data can be encoded and stored. Fixed disks in the form of hard disks are a common component of a computer’s storage system, but most other forms of disk hardware (floppy disks, CD-ROMs, etc.) have become obsolete.

Accessing data from a disk is not as fast as accessing data from main memory, but disks are much cheaper. And unlike RAM, disks hold on to data even when the computer is turned off. Consequently, disks have historically been the storage medium of choice for most types of data, but are slowly being replaced by newer forms of storage like flash drives, solid state drives, and cloud storage.

Disk vs. disc

Like several other common spelling variations, most of the disk/disc distinction comes from differences in American English and British English spelling conventions. In the United States, “disk” is the common usage, whereas “disc” is preferred in the United Kingdom.

There are also distinctions that are relevant to biology and computer hardware, regardless of geographic location. In medical settings, “disc” should be used in all anatomical references (optic disc, spinal disc, etc.). In computer engineering, “disk” is the preferred spelling, except in audio/visual contexts like disc jockey (DJ), compact disc (CD), or digital versatile disc (DVD).

Types of disks

There are two basic types of disks in the computer hardware realm: magnetic disks and optical disks.

Magnetic disks

Data is encoded to magnetic disks using magnetized polarities; this means flux reversals between positive and negative domains on the disk are interpreted as binary data. A user can record and erase data on a magnetic disk any number of times. Magnetic disks come in a number of different forms:

  • Floppy disk: A typical 5-inch floppy disk can hold 360KB or 1.2MB. 3-inch floppies normally store between 400KB 1.44MB of data. Floppy disks are obsolete today, but are still referenced in some modern iconography.
  • Hard disk: Hard disks can store up to 20TB of data. They are still used in modern computer systems, although solid state drives (SSDs) are more common.
  • Removable cartridge: Removable cartridges are portable hard disks encased in a metal or plastic cartridge. Removable cartridges are fast, though usually not as fast as fixed hard disks.

Optical disks

Optical disks record data by burning microscopic holes in the surface of the disk with a laser. To read the disk, another laser beam shines on the disk and detects the holes by changes in the reflection pattern. Optical disks come in three basic forms:

  • CD-ROM: Stands for Compact Disc-Read Only Memory. Most optical disks are read-only, meaning they come pre-filled with data. Users can read the data from a CD-ROM, but they cannot modify, delete, or write new data.
  • WORM: Stands for Write-Once, Read-Many. WORM disks can be written once and then read numerous times; however, a special WORM disk drive is needed to write data onto a WORM disk.
  • Erasable optical (EO): EO disks can be read to, written to, and erased just like magnetic disks.

What is disk usage?

Not to be confused with capacity, disk usage is the percentage of storage that is occupied at any given time. Disk capacity-or disk space-is the total space that the disk maintains, including what is used and unused. Depending on the type of disk and how it is being used, too much disk usage can impede overall computer performance. Similar to capacity, disk usage is measured in kilobytes (KB), megabytes (MB), gigabytes (GB), and terabytes (TB).