Term: sector

A sector is a fixed-size “chunk” of information as stored on magnetic or other media such as hard disks, optical discs, and flash-based memory.

Information on a hard disk is divided into:

• Platters: the entire surface of one side of one or more of the metal disks in the drive. Each disk is coated with magnetic material used to hold individual bits of digital information. (The entire image at right represents one platter).
• Tracks: one circle of information on the magnetic media. A platter may contain many concentric tracks of information. (The red ring labelled “A” is a track.)
• Sectors: one group of information within a track. Sectors on magnetic disks typically represent the way that the information is stored. A track would be comprised of several sectors with essentially “empty” space inbetween them. (The portion of the “A” track that’s labelled “C” is a sector.)

A sector most commonly contains 4096 bits, or 512 bytes. (Newer hard drives may use 32,768 bit – aka 4096 byte – sectors, but for compatibility with older interfaces and operating systems, may also expose a simulation of 512 byte sectors.)

Sectors are accessed as single units at the hardware level. This means that when reading, the entire sector is read, regardless of how much of its contents are required. Similarly, when writing data, it’s the entire sector that’s written, regardless of how much information in it is to be changed.

(Image courtesy of Wikimedia Commons)