External hard drives are external peripherals that comprise a standard desktop or notebook hard drive contained within a portable enclosure that provides one or more types of interface connector to link the external hard drive to a desktop or notebook computer. Many external hard drives include backup software that features a "one-button" means of backing up data from your internal hard drive.
External hard drives may use any of three interfaces:
USB 2.0 is by far the most common interface supported by external hard drives. USB 2.0 nominally provides 60 MB/s bandwidth, but overhead typically reduces this to an effective 25 MB/s to 30 MB/s. Because standard hard drives can use 50 MB/s or more bandwidth, the USB 2.0 interface throttles throughput noticeably for the fastest external hard drives, making them "feel" somewhat slower than an internal drive. The advantage of USB 2.0 is that it is ubiquitous, so a USB 2.0 external hard drive can be connected to nearly any notebook or desktop system.