I wouldn't recommend a MyBook Digital.. They don't last very long, and they have the weird habit of (when bit shocking) losing data.. I've also recently purchased an External HDD, I bought this Iomega USB (200GB)..
http://www.priceguidenetwork.com/images/T/33703-01.jpg
It's very small, has steel casing and it won't break even if you drive over it with a truck.. Adding more to that, its super shock resistant, so when you're traveling a lot with it (I take it everywhere with me) It's a must, were most Portable HD's let you down.. It also runs at 5400RPM (Same as a laptop HD, about the same transfer speed) The only downside is you get 300GB less for the same money
If you want to use it only at home (IE; Adding more drive space to your computer) I would recommend purchasing an Internal HD because Data transfer over USB is much (much) slower..
Hope this was usefull ^^