A hard disk drive or hard drive is a computer data storage device which magnetically reads and writes non-volatile (it can maintain data storage after power is removed) data. Sometimes confused with memory, or RAM, HDDs are usually today's computer's primary data storage component.

HDDs come in many form factors (referring to the physical width of the drive (2.5", 3.5", etc.) and fit into bays inside of the case. They connect to the mainboard of the computer via SATA (3 GB/s, 6 GB/s), PATA, or SAS interfaces- the most common of these is SATA, in more recent computers. HDDs also utilize mechanized moving internal parts and thus have differing speeds which will affect read/write times which ultimately effects computer performance- these speeds are typically measured in RPM.

Multiple HDDs may be installed on a machine and may be utilized for the boot (or the Operating System like Windows or Mac OS) or for secondary storage. They also may be configured in one of several RAID (Redundant Array of Independent Disks) configurations which can increase performance or reliability.