hard disk drive (HDD)

A computer hard disk drive (HDD) is a non-volatile memory hardware device that controls the positioning, reading and writing of the hard disk, which furnishes data storage. Hard disk drives are commonly used as the main storage device in a computer. HDDs often store operating system, software programs and other files, and can be found in desktop computers, mobile devices, consumer electronics and enterprise storage arrays in data centers.

A hard disk drive -- often shortened to hard drive -- and hard disk are not the same things, but they are packaged as a unit and either term can refer to the whole unit.

In a computer, an HDD is commonly found in the drive bay and is connected to the motherboard via an ATA, SATA or SCSI cable. The HDD is also connected to a power supply unit and can keep stored data while powered down.

Hard disk drive components and how it works

Most basic hard drives consist of a number of disk platters that are positioned around a spindle inside a sealed chamber. The chamber also includes read-and-write heads and motors. 

The motor is used to spin the platters, which hold the data, at up to 15,000 rotations per minute (a higher rpm number results in faster performance). As the platters spin, a second motor controls the position of the read-and-write heads that magnetically record information to, and read information from, tracks on each platter.

External HDDs

Most HDDs are found internally in a computer and work as stated above. However, individuals can also purchase external hard drives. External hard drives can be used to expand the storage capacity, or act as a portable place to back data up to. An external hard drive can connect to a computer or device through a USB 2.0 interface or with eSATA. External hard drives may also have slower data transfer rates compared to internal HDDs.

History of hard disk drives

The hard disk was created in 1953 by engineers at IBM who wanted to find a way to provide random access to high capacities of data at a low cost. The disk drives developed were the size of refrigerators, could store 3.75 megabytes of data and began shipping in 1956. Memorex, Seagate and Western Digital were other early vendors of hard disk drive technology.

Hard disk drive form-factor size has continued to decrease as the technology evolves. By the mid-1980s, 3.5-inch and 2.5-inch form factors were introduced, and it was at this time they first became a standard in personal computers (PCs).

Hard disk drive density has increased since the technology was first developed. The first hard disk drives were able to store megabytes of data, while today they are in the terabyte (TB) range. Hitachi released the first 1 TB hard drives in 2007. In 2015, HGST announced the first 10 TB hard drive.

HDD technology developments

In 2013, Seagate announced hard disk drives that use shingled magnetic recording (SMR) technology. SMR increases storage density in hard disk drives by layering the magnetic tracks on each disk, rather than placing them parallel to each other. It is referred to as shingled because the tracks overlap similar to shingles on a roof.

Hard disk drive components
Hard disk drive components

HGST announced the first helium-filled hard disk drive in 2012. Helium is less dense, cooler and lighter than air, and can, therefore, consume less power, increase drive density and improve performance compared with traditional hard disk drives. In 2016, Seagate announced its own 10 TB helium hard drive.


The main alternative to hard disk drives in PCs and the enterprise are solid-state drives (SSDs). HDDs are now starting to be replaced by SSDs.

Unlike hard disks, SSDs contain no moving parts. SSDs also have lower latency than HDDs, and therefore are often favored to store critical data that needs to be accessed quickly and for applications with a high input/output demand. SSDs are configured to deliver high read and write performance for sequential and random data requests. Additionally, SSDs don't store data magnetically, so the read performance remains steady, regardless of where the data is stored on the drive.

However, SSDs are more expensive than HDDs from a price-per-gigabyte standpoint. Many enterprise storage arrays ship with a mix of HDDs and SSDs to reduce costs while providing better performance. SSDs also have a set life expectancy, having a finite number of write cycles before performance slows. This detrition happens faster than how long it takes for an HDD to fail over time.

This was last updated in August 2019

Continue Reading About hard disk drive (HDD)

Dig Deeper on SAN technology and arrays