I've got a 20gb laptop HD mounted in an external case - USB connection.
This one. I use it to back up files on my laptop and to also transfer files from my desktop to my laptop. It works very well. There's no reason why you can't do the same and you'll have a back up unit as a result.
What prompted me to do this was a fried laptop in the first place. I have a friend that worked for a PC consultant at the time and he had one of their service guys connect my soon to fail laptop HD via an adapter to a desktop and the files were copied in time. It was then I decided I needed a better back up system, which is why I did the above.