I tried to do my normal update today and then it said I didn't have enough room on my external, I got real frustrated and deleted everything off the external. So now I am trying to back everything up again. I am using 432GB's on my iMac, on my external I have 465GB of free space. However, when I go to run the Time Machine it says: "This backup is too large for the backup volume. The backup requires 510GB but only 465GB are available." How can this be? What do I need to do to back up everything again? Your help is very much appreciated. Thank you.