Probably an internal drive inside an external enclosure. Cheapest is bad...the HD is already OUTSIDE your computer - don't need to skimp on safety. I recommend the Vantec NexStar combo enclosure.
http://www.tigerdirect.com/applicati...137&CatId=1204
Very compact, Firewire is good, and the base to mount it vertically is very stable. Both of the plates on the sides are aluminum, which act as heatsinks. I have a 160GB Maxtor in mine, nice and quiet (compared to my prebuilt Western Digital external).