I have a Toshiba 750GB USB external hard drive. I have a 320 GB internal hard drive on my MacBook Pro which I purchased in September 2011. I used it for Time Machine but for some reason Time Machine ceased to function, taking weeks instead of hours to back up. So I cleared all the files and have been backing up manually, i.e. once a week deleting the files and then saving all my files again. Everything is in one folder so easy to do, takes about 2 hours for about 180GB. Then suddenly, my external hard drive says that it has only 68 GB free, although there are no files on it. I used Disk Utility to erase free space, but didn't get any more free space. Can anyone clue me in? Do I have some sort of partition on the hard drive? I would like to restore it to being utterly blank.