I have 50GB on my main drive. My external back up drive is 75GB. The initial bu takes up the 50 GB as expected. After about 4 days it says it cannot do any more back up because there is not enough space. I have not added anything to the HD and have made changes to very little. It says it needs 21GB to do the backup. That implies that almost half of what is on the main drive needs to be backed up. 1. I thought after the first backup the subsequent bu would only back up what is new or changed? 2. I thought it would delete the older backups to make room? What am I missing here?