A track is any of the concentric circles on the magnetic coating on a platter or floppy disk over which one magnetic head passes while it is stationary but the platter or floppy is spinning. In the case of CDROMs, DVDs and other optical disks, there is a single spiral track formed in a plastic coating.
The magnetic coating, which is used to store data, is formed through the deposition of a high precision magnetic material and is typically only a few millionths of an inch thick. Modern hard disk drives (HDDs) typically contain multiple platters, each with its own set of magnetic heads (one for each side) for recording, reading and erasing data.
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. The thinner the tracks, the greater the storage capacity of the disk. An important trend with regard to HDDs has been the steady reduction in track thicknesses, which has been made possible by improvements in magnetic materials, in magnetic heads and in head control mechanisms.
The density of tracks (i.e., how close together they are) is measured in terms of tracks per inch (TPI). It varies widely according to the type of disk and when it was produced. A typical floppy disk has a track density of 135tpi, whereas modern HDDs have densities of thousands of tracks per inch.
A single, imaginary, concentric circle that cuts through all of the platters in a HDD and includes the same track on each side of each platter is called a cylinder. The number of cylinders in a HDD is, of course, the same as the number of tracks on any platter in that HDD. It is useful to be aware of cylinders when installing some operating systems, particularly the BSD systems (e.g., FreeBSD and NetBSD).
Tracks are divided into a number of segments called sectors. Each sector 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).
Tracks on each platter are assigned consecutive numbers by the operating system, beginning with zero for the outermost track. The operating system remembers where data is stored by noting these numbers as well as those of the cylinders, heads and sectors.
Created March 5, 2006. Updated July 14, 2006.