If you are looking at the CD in Windows Explorer, it will aways report bytes used and 0 bytes free. When you next perform a backup in OneCare, it will ask for the last CD of the backup set and will require a new one if there is not enough room available on that CD. The maximum amount of data that can be stored on the disc will range from 650 megs to 800 megs, depending on capacity, but will always show 0 bytes free in Windows Explorer.
-steve