I have a 120 GB hard drive, with about 22 GB used, and am backing up to an 80 GB external hard drive. It has about 24 GB used. I cloned the hard drive, and then elected to do a differential backup daily. Eventually, this will fill up my external drive. Can I start over every month or so (erase everything, do a new clone and start backups again)? Can this be done automatically? Any suggestions?