Disk Geometry

The Hard disks are made of one or more magnetic disks, known as PLATTERS that rotate around a central shaft. Each platter holds millions of magnetic particles, whose magnetic field determines the storage of  the information.

                                                 Boom     Head Sector   Spindle   Track    Platter

For each platter there is a movable arm from the outer edge of the disk to the central shaft. At the end of every arm is a read-write head which could access any point on the disk's surface. All the arms are connected in the Figure together so in which they move in unison.

Every platter holds a set of blocks or sectors that are used to store data. A Sector is the smallest unit of one platter.   One   sector   holds   512   bytes.   That   blocks   are   organized   into   circular   rings   known as   tracks.   A group of sectors both form a track.

Tracks on different platters that are situated at the same distance from the edge of their respective disk platter, are combined to form a cylinder. That is diagrammatically represented in the next Figure

Because all the tracks of a cylinder are accessed through the read-write head at the similar time, the delay in accessing the data stored on different platters but in the similar cylinder is much lesser than if the data is located in different cylinders.