MacResQ is having a sale this week on the 2.5" Archos 20gig FW hdd. I own one myself and it's very portable. The only thing I don't like is the plug that plugs into the drive is kinda large. But the drive is only $99 right now, so it's totally worth it. I haven't had any problems with it so far, and I bought a neoprene PDA case to protect it.
Incidentally, I ordered a USB2.0 version, and the drive I got had a cord for USB 2.0, and a Firewire cord. So I would assume that if you order a FW version you'll probably get a USB cord as well.