hi, i have a macbook with an internal drive of which about 230gb if full and used. I have a 250gb external which i have been using to back up with time machine, done it several times over a few months. Now there is like 70gb left on the external, and when i try to back up, it says failed, not enough space on external. But i thought when there is not enough space, it simply deletes and backs over older time machine backups. Can anyone help? I dont wanna fork out for another external again. thanks.