How a hard disk drive works

A hard disk drive stores data magnetically on one or more rigid platters that spin at high speed. A read/write head, mounted on an arm that moves across the surface, magnetises tiny regions to record bits and senses their magnetisation to read them back. Because the head floats just above the spinning platter on a thin cushion of air, the drive can pack a great deal of data onto each surface while accessing any part of it quickly, which is what made the hard disk the main storage of personal computers for decades.

The surface is organised so the data can be located precisely. Concentric rings called tracks are divided into sectors, and the matching tracks across all platters form a cylinder; clusters group sectors into the units a file system allocates. To find a given block, the drive positions the head over the right track and waits for the desired sector to rotate beneath it. Older addressing schemes described locations by cylinder, head, and sector, and a translation step (CHS translation) reconciled this with what the interface reported. Drives connect through interfaces such as ATA, also called IDE.

Before a disk can hold an operating system, it must be partitioned and prepared. The first sector of the disk holds the master boot record (MBR), which contains a small boot program and the partition table describing how the disk is divided. When the computer starts, the firmware reads this sector, runs its code, and uses the partition table to find the active partition and its boot sector, which continues loading the operating system. This chain — MBR, partition table, then partition boot sector — is central to how a PC starts from its disk.

Frequently asked questions

How does a hard disk store data?

It records data magnetically on spinning platters, with a read/write head magnetising small regions to write bits and detecting their magnetisation to read them back.

What is the master boot record?

The master boot record is the first sector of a disk; it holds a small boot program and the partition table that describes how the disk is divided, and the computer reads it when starting up.

What are tracks, sectors, and cylinders?

Tracks are concentric rings on a platter, sectors are the divisions of a track, and a cylinder is the set of matching tracks across all platters, together giving each block a precise location.