I have a 11inch base model MBA. I'm thinking a 80GB external HD should be enough to back up the contents stored on my 64GB SSD by using Time Machine. I did a price check and found out a 320GB external HD is not that much more expensive than a 80GB one. So I'm just wondering if I format the bigger storage HD to use it for Time Machine, can I use the leftover space to store other stuff? Or the external HD has to dedicated to my MBA backup only? Thanks in advance.