I plugged my external drive into my mac, and told time machine to use it as a backup disk. Then time machine said that I had to erase everything on the harddrive. So I canceled, and backed up everything on my external harddrive. Then I told time machine to erase everything on the drive. Now time machine dont start backing up. It says: Oldes Backup: -- Latest Backup: -- Next Backup: -- What to do?