Please recommend me a good external hard drive for Time Machine backup... i now only have about 20gb of data to back up, which i think will increase, but maybe double or triple. so i'm thinking 120-250gb? please recommend a cheap solution because i don't want to spend too much since i don't have that much data to back up. also, something with a small footprint would be nice, too, since i don't have much space in my apartment. thanks!