Hard drive

A hard disk drive (sometimes abbreviated as Hard drive, HD, or HDD) is a device used to permanently store and also retrieve information. There are many variations, but their sizes are generally 3.5" and 2.5" for desktop and laptop computers respectively. A hard drive consists of one or more platters to which data is written using a magnetic head, all inside of an air-sealed casing. Internal hard disks reside in a drive bay, connect to the motherboard using an ATA, SCSI, or SATA cable, and are powered by a connection to the PSU (power supply unit). The images below show the components of a hard drive inside of both desktop and laptop computers.

Tip: The hard disk was first introduced on September 13, 1956.

Hard drive components

Inside a computer hard drive

As can be seen in the above picture, the desktop hard drive consists of the following components: the head actuator, read/write actuator arm, read/write head, spindle, and platter. On the back of a hard drive is a circuit board called the disk controller.

Tip: New users often confuse memory (RAM) with disk drive space. See our memory definition for a comparison between memory and storage.

How is data read and stored on a hard drive?

Data sent to and from the hard drive is interpreted by the disk controller, which tells the hard drive what to do and how to move the components within the drive. When the operating system needs to read or write information, it examines the hard drive's File Allocation Table (FAT) to determine file location and available write areas. Once they have been determined, the disk controller instructs the actuator to move the read/write arm and align the read/write head. Because files are often scattered throughout the platter, the head needs to move to different locations to access all information.

All information stored on a traditional hard drive, like the above example, is done magnetically. After completing the above steps, if the computer needs to read information from the hard drive, it would read the magnetic polarities on the platter. One side of the magnetic polarity is 0, and the other is 1. Reading this as binary data, the computer can understand what the data is on the platter. For the computer to write information to the platter, the read/write head aligns the magnetic polarities, writing 0's and 1's that can be read later.

External and Internal hard drives

Although most hard drives are internal, there are also stand-alone devices called external hard drives, which can backup data on computers and expand the available disk space. External drives are often stored in an enclosure that helps protect the drive and allows it to interface with the computer, usually over USB or eSATA. A great example of an external backup device that supports multiple hard drives is the Drobo.

Adaptec laptop hard disk drive enclosureExternal hard drives come in many shapes and sizes. Some are large, about the size of a book while others are about the size of a cell phone. External hard drives can be very useful since they usually offer more space than a jump drive and are still portable. The picture to the right is an example of a laptop hard disk drive enclosure from Adaptec. The user may install any size of laptop hard drive they desire into the enclosure and connect via USB port to the computer.

HDD being replaced by SSD

Solid State Drives (SSDs) have started to replace hard disk drives (HDDs) because of the distinct performance advantages they have over HDD, including faster access times and lower latency. While SSD is becoming more and more popular, HDD will continue to be in desktop computers with SSD because of the available capacity and value per dollar that HDD offers over SSD.

Other help and related links

Also see: Hard drive terms, Hybrid hard drive, Storage device, VHD