An external hard disk drive is basically a hard disk, usually a 2.5” or 3.5” drive – exactly like those found inside desktops and laptops, that is installed within a specially designed enclosure or chassis which allows the hard disk to work outside a computer. The chassis will power the hard disk through either power tapped from the connection with the host computer or an external power supply. 3.5” drives would need external power. The external hard disk is connected to the computer via a USB or a firewire cable.

There are some desktop casings that allow for hard disk drives to be easily installed or removed without opening up the main casing of your computer.

Newer enclosures allow the hard disk to be used without being linked to a computer. Such enclosures can be used for the uploading of digital photographs, be used as a media player for movie or audio files of many different formats and even allow the hard disk to be used as network-attached storage (NAS).

Besides the types of failures that would happen to hard disk drives installed in computers, external disks are more prone to impact damage because they more liable to get knocked over or dropped while in transit. Hard disks have an operational speed of usually between 5,400 revolutions per minute to 15,000rpm. With the read-write head hovering at only xx above the hard disk platter

Also the connection to the computer and the power supply also introduce additional risk factors. A broken, loose or corroded connection would mean that a computer would not be able to connect to or recognise the hard disk or, worse, the connection is intermittent, data on the hard disk may get corrupted.