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[ The PC Guide | Systems and Components Reference Guide | Hard Disk Drives | Construction and Operation of the Hard Disk | Hard Disk Platters and Media ]

Tracks and Sectors

Platters are organized into specific structures to enable the organized storage and retrieval of data. Each platter is broken into tracks--tens of thousands of them--which are tightly-packed concentric circles. These are similar in structure to the annual rings of a tree (but not similar to the grooves in a vinyl record album, which form a connected spiral and not concentric rings).

A track holds too much information to be suitable as the smallest unit of storage on a disk, so each one is further broken down into sectors. A sector is normally the smallest individually-addressable unit of information stored on a hard disk, and normally holds 512 bytes of information. The first PC hard disks typically held 17 sectors per track. Today's hard disks can have thousands of sectors in a single track, and make use of zoned recording to allow more sectors on the larger outer tracks of the disk.

A platter from a 5.25" hard disk, with 20 concentric tracks drawn
over the surface. This is far lower than the density of even the oldest
hard disks; even if visible, the tracks on a modern hard disk would
require high magnification to resolve. Each track is divided into
16 imaginary sectors. Older hard disks had the same number of
sectors per track, but new ones use zoned recording with a different
number of sectors per track in different zones of tracks.

A detailed examination of tracks and sectors leads into a larger discussion of disk geometry, encoding methods, formatting and other topics. Full coverage of hard disk tracks and sectors can be found here, with detail on sectors specifically here.

Next: Areal Density

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