some external hard drives have external power as well,
so they do not drain the computer battery. they are a
great tool for holding large files...some videographers
use them to store movies to free up the main hard drive.
For this use you would want one that has a fire wire
connection. as well as the USB. see this comparison:

http://reviews.cnet.com/hard-drives/

I also have a "sled" that contains an auxilliary
hard drive which is exchangeable with the cd/dvd
disc drive .