Are these DVD RW discs or DVD R discs?
If the former, have they been erased previously? OneCare doesn't work well with RW discs, especially if they are re-used.
It could be a media issue if these are R type discs. Is it name brand media?
You may want to try resetting the backup plan since you are actually starting a new, full backup anyway.
See this post for how:
http://social.microsoft.com/Forums/en-US/onecarebackupandrestore/thread/5f18a60d-add4-4ff2-87cf-021db7e81917If the reset and/or different discs doesn't resolve it, 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 Forum Moderator