Hard disk

File:Hard disk WD 400.jpg
Top and bottom views of a Western Digital WD400 3.5" hard disk

A hard disk drive (HDD), hard disk or hard drive, is a data storage device for computers which uses magnetic storage to store data. The capacity of a hard drive is usually measured in gigabytes (GB), however hard disk capacity can also be measured in terabytes when the capacity is over 1000 gigabytes. A gigabyte is one thousand megabytes and a megabyte is one million bytes, which means that a gigabyte is one billion bytes. Some hard drives are so large that their capacity is measured in terabytes, (TB) where one terabyte is a thousand gigabytes (1 TB = 1024 GB).

Different interfaces

File:Serial ATA1 Data Cable.jpg
Serial ATA cables are a common way to connect hard disks

Over the years there have been many disk interface types, though all used the same rotating platter recording technology. Differences were in how the data was encoded to binary, data integrity, data transfer speeds, cabling requirements, and cost. In 2009, it was common to attach a hard disk using a Serial ATA connection. The connection that came before that was called "IDE" and is called Parallel ATA today. In large data centers, Fibre Channel is often used.

For servers, the SCSI interface is very popular. There are several types and versions of SCSI interface, like parallel and Serial Attached SCSI, each stepping-up in terms of speed and price. Within servers, several SCSI drives are often used in conjunction with each other, in order to safeguard against data loss or corruption (this is known as RAID - and there are many configurations to choose from).


An HDD has a disk motor and an actuator motor that positions the read/write head. Wires from the actuator connect to the read/write head to amplifiers. They are bridged by the head support arm. In modern drives, acceleration at the head reaches 550 g, so a head support arm connects the actuator and the read/write head.

The actuator controls the read/write head and acts as a permanent magnet. A metal plate supports a squat neodymium-iron-boron (NIB) magnet. Beneath this plate is the voice coil, which is attached to the actuator hub. Beneath the actuator hub is a second NIB magnet, mounted on the bottom plate of the actuator.

The voice coil is shaped like an arrow. It is made up of a magnet with plastic insulation. This magnet interacts with the actuator magnet, causing the disk to move. If the magnetic field were uniform it would cancel out, but the surface magnet is divided between north and south poles in the middle so force is produced instead of canceled out.

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