So I have Time Machine syncing with an external drive and its been backing up perfectly without a problem for the last month. I have not plugged in the external drive in a week, but when I plugged it in to backup today, time machine will not back up to it. The computer sees that the drive is there and identifies at as the Time Machine drive but will not backup to it. The external drive is not full and there is no damage on it. Also, in the time machine system preferences window, the oldest, last, and next backup dates are all blank, they were filled before. How can I fix this problem? Thanks in advance.