An external har disk drive is a type of hard disk drive which is externally connected to a computer, and may be portable. External hard disk drives are connected to the computer using a cable which is compatible with Small Computer System Interface (SCSI), Integrated Drive Electronics (IDE), Universal Serial Bus (USB), IEEE 1394 (Firewire), CAT-3 or CAT-5 cable (Ethernet), eSATA or other bus standards.