You need to look for a "NAS" device..Network Attached Storage devices. Make sure it has Ethernet connection (it should to be a NAS) but there are some external drives with USB only.
here are a couple of examples from DABS and Maplins:
http://www.dabs.com/productview.aspx...avigationKey=0
http://www.maplin.co.uk/module.aspx?...18055&doy=27m8