Time Machine is reporting the destination drive is full and asking me to allow it to delete the oldest backup done only a week ago, yet the drive reports it has 19.88 GB free. I have a 175 GB partition on an external HD backing up about 100 GB of my MacBook Pro (OS-X; V. 10.5.8). I thought TM backed up weekly and monthly but I am getting nowhere near that. How is it that TM is not using 20 GB of this free space?