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. Newer hard disks introduced in late 2009-2010 use an Advanced Format, which stores 4kb of information per track (eight times more). 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 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