A hard disk drive (HDD) is a nonvolatile storage device used in computers that persists data in a durable way. IBM introduced HDDs in 1956, and they have since become the dominant secondary storage device for general-purpose computers.


Let’s take a look at the different components of an HDD and their roles.


An HDD consists of one or more circular magnetic plates, called platters, stacked over each other to hold the data. Each platter consists of a nonmagnetic material such as aluminum, glass, or ceramic, coated with a shallow magnetic material 10–20 mm in depth and an outer layer of carbon for protection. Each platter has two working surfaces, one on either side, which holds the actual data.


The stack of platters is held together by a spindle motor, which is responsible for controlling and rotating the platters. Most HDD failures are related to the spindle's inability to spin the platters.


Each platter has an actuator arm positioned on top of it, which is responsible for reading, writing, and deleting the data on its respective platter. Actuator arms are also called read-write arms.

Read-write arms have two separate heads:

  • The read head: Responsible for reading the direction of magnetic current and converting it into bits.

  • The write head: An electromagnet with a coil shape that magnetizes the bits to north facing up (0s) and north facing down (1s) by changing the direction of the current.

An actuator holds the actuator arms responsible for positioning the read-write heads on the spinning platters.

Track, sector, and cylinder

Each working surface of the platter includes thousands of concentric rings called a track. Each track can hold a large amount of data. Tracks start at 0 outside the platter and go on, increasing to the inner side.

Each track includes thousands of smaller units called sectors. A sector is the fundamental unit of storage on an HDD. The data size of a sector is always a power of 2. A sector can either store 512 or 4096 bytes of data. Each sector contains a sector identifier, actual data, and self-correcting error codes for detecting and correcting the corrupted data. Each track in a platter includes the same number of sectors. However, the platter packs sectors more tightly near the center than around the peripheral.

A cylinder is any set of tracks of equal diameter in an HDD. We can visualize the cylinder as a single, imaginary circle that cuts through the platters.

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