Sector Definition

A sector is a segment of a track on a hard disk drive (HDD) or a floppy disk. It generally contains 512 bytes and is the smallest unit of data that can be accessed by a disk drive (although software makes it possible to access individual bytes and even individual bits).

A track is any of the concentric circles on the magnetic media on a platter in a HDD or on a floppy disk over which one magnetic head (i.e., a device used for reading and writing data on the disk) passes while the head is stationary but the disk is spinning. A platter is a thin, high precision aluminum or glass disk that is coated on both sides with a high precision magnetic material; most HDDs contain multiple platters, all mounted on a single shaft that is rotated directly by a spindle motor. Each track on a modern HDD has a width of only a few microns (i.e., millionths of a meter), and there can be tens of thousands of tracks on each platter.

When a HDD or floppy disk is given a low level formating, it is divided into tracks and sectors. The operating system and disk drive remember where data is stored on a disk by recording its track and sector numbers.

Modern HDDs employ a technique called zoned bit recording (ZBR) to assign more sectors to outer tracks on disks than to inner tracks in order to take advantage of the greater circumferences of outer tracks and thereby increase the storage capacity of disks.

In the IBM personal computer architecture, the first 512 byte sector of a disk contains the master boot record (MBR), which is a small program that is executed when a computer boots up (i.e., starts up) in order to find the operating system and load it into memory.

A bad sector is a sector that cannot be used due to a physical flaw on its magnetic material. When a disk is formatted, the operating system identifies any bad sectors on it and records their location so they will not be used. It is not uncommon for newly manufactured HDDs to have a few bad sectors, although this does not affect their overall performance. If a sector that already contains data becomes damaged, special software is required to recover the data.

Created February 1, 2006. Updated July 10, 2006.
Copyright © 2006 The Linux Information Project. All Rights Reserved.