A hard disk drive (sometimes abbreviated as Hard drive, HD, or HDD) is a non-volatile memory hardware device that permanently stores and retrieves 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).
A hard drive can be used to store just about any type of data, including pictures, music, videos, and text documents. Computers have a hard drive and use it to store files for the operating system and software that run on the computer, as well as files created or downloaded to the computer by a user.
The amount of data a hard drive can store depends on the storage space of the hard drive. Older hard drives had a storage size of several hundred megabytes (MB) to several gigabytes (GB). Newer hard drives have a storage size of several hundred gigabytes to several terabytes (TB). Each year, new and improved technology allows for increasing hard drive storage sizes.
Hard drive components
As can be seen in the picture above, 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.
Note: The above picture is an example of a traditional hard drive and not an SSD.
How is data read and stored on a hard drive?
Data sent to and read 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.
External 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 a laptop hard drive of any storage capacity into the enclosure and connect it 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 SSDs is becoming more and more popular, HDDs continue to be used in many desktop computers largely due to the value per dollar that HDDs offer over SSDs. However, more and more laptops are beginning to utilize SSD over HDD, helping to improve the reliability and stability of laptops.
History of the hard drive
The first hard drive was introduced to the market by IBM on September 13, 1956. The hard drive was first used in the RAMAC 305 system, with a storage capacity of 5 MB and a cost of about $50,000 ($10,000 per megabyte). The hard drive was built-in to the computer and was not removable.
In 1963, IBM developed the first removable hard drive, having a 2.6 MB storage capacity.
The first hard drive to have a storage capacity of one gigabyte was also developed by IBM in 1980. It weighed 550 pounds and cost $40,000.
1983 marked the introduction of the first 3.5 inch size hard drive, developed by Rodime. It had a storage capacity of 10 MB.
The first solid-state drive (SSD) as we know them today was developed by SanDisk Corporation in 1991, with a storage capacity of 20 MB. However, this was not a flash-based SSD, which were introduced later in 1995 by M-Systems. These drives did not require a battery to keep data stored on the memory chips, making them a non-volatile storage medium.
Other help and related links
- How to erase a hard disk drive and reinstall the operating system.
- Determining available hard drive space.
- Installing a PC IDE/EIDE hard drive.
- Listing of computer hard drive manufacturers.
- Computer hard drive help and support.