This can happen for two main reasons.
1. The disc is an RW or rewritable disc format and may have been erased. OneCare backup doesn't like RW discs for some reason and a previously used disc gives it even more fits. Try the cheaper type R discs.
2. Some other software is conflicting with the backup program's ability to access the CD/DVD writer. Packet Writing software should be uninstalled. Both Roxio and Nero include packet writing software by default in their suits - DirectCD and InCD are the programs. These programs monitor the drive and may be holding the drive.
If neither of the above apply, please contact support.
How to reach support (FAQ) - http://social.microsoft.com/Forums/en-US/onecareinstallandactivate/thread/30400b52-7f26-4ba0-bc18-17e305329d90
-steve
Microsoft MVP Windows Live / Windows Live OneCare, Live Mesh, & MS Security Essentials Forums Moderator