External Hard Drives normally comes with USB, sometimes Fire wire but on the rare occasion something called eSATA. Basically it goes like this in terms of speed:
USB 1.1 –> USB2 -> Firewire 400 –> Firewire 800 and finally eSATA which is the same speed as connecting the drive internally. I have not seen many FireWire 800 cases around and even FW400 commands a premium. Of course for the Mac User you have little choice, Apple have not got any eSATA connectors on any of there machines.
