I've been using the same 500gb external drive since last September for my iMac, also with 500gb. A couple weeks ago I was getting an error saying that there was not enough room on the external drive to backup my computer. I posted here, and I was told to erase old backups. So I did that. Now, I'm getting another strange error saying that the external drive requires over 500gb to complete the backup. I don't see how this is possible since the my computer doesn't even hold that much. Any suggestions?