I made a full backup of my C: drive to an external drive O: Several weeks later I made an incremental backup according to instructions received from Acronis Tech support. Again, several weeks later I made another incremental backup. Everything seemed OK until the other day I tried another incremental backup and received an error message that the drive was full. I then checked my capacities and my c: drive has a total capacity of 144GB with 83GB used and 72GB free. My external O: drive has a capacity of 156GB with 153GB used and only 2.8GB free. How can this be? Shouldn't my backup be pretty much the same as my C: drive? The backup is nearly double the original. What's up?