I usually like putting my own external together, it's quite easy, just grab a bay for about £20 and then whatever hard drive you want, that way you can actually get a good quality hard drive.
The thing about external hard drives is unless you have an external SATA port on your computer, it's going to have to run through a USB or Firewire port, which weren't really designed to run entire program through, lots of different data passing very quickly through them. If you've got a slow hard drive (5400 RPM or less), it'll probably be better I guess though. Though Windows and certain programs may have issues running from a removable drive. I'd do a bit of research first if I were you certainly.
