I've been reading about time machine and thinking it's time to get grips with a back up. The info I've read states that the external hard drive that time machine backs up to should be bigger than the disk that is being backed up. What I'm not clear on is this: I have a 250gb disk on my macbook but am only using around 20gb of it so far, so does time machine just back up the file system and folders in use? i:e, the 20gb, in which case I can't see that the external hard drive being smaller would be a problem initially, maybe in time as more is added to the drive, but not for now? If so, could I get away with using a 160gb external hard drive? I have a 'spare' one that I can use if I can get away with it so I don't want to buy a new one if it's not necessary. Thanks.