My wife uses time machine on her macbook pro running mountain lion, backing up to a 320 gig external drive. It says there are only 20 gigs available and it needs 30 to continue to back up. It's been doing this since June - she only just noticed this. I thought Time Machine would fill up a hard drive, more or less, and when it runs out of room, will start to erase the oldest backups. Is that not the case? Can you run out of room on a disk with Time Machine. Or is there a setting we can use to allow it to continue to back up? thanks.