An external /removable hard drive is a type of hard disk drive which is connected to a computer with a single high-speed interface cable, usually with plug-and-play interfaces such as USB or FireWire. An external hard drive sits outside the main computer case in its own enclosure. The enclosure is slightly larger than the hard drive itself, and sometimes contains a cooling fan.

An external /removable hard drive is quite useful. Internet access regularly exposes computers to potential security threats like Trojan horses, viruses and spyware. With an external /removable hard drive, users can back up or store important information separate from the main internal hard drive, which can guarantee security of data. Another advantage of an external hard drive is that it is portable and operates on a plug-and-play basis, so data on it can be carried between computers.

See: SD Card, CF Card, iPod, iPhone