Not sure why you posted to the forum you went to instead of going through OneCare Help to find this forum, but...
See this post for how to reset the backup plan -
http://social.microsoft.com/Forums/en-US/onecarebackupandrestore/thread/5f18a60d-add4-4ff2-87cf-021db7e81917It would appear that OneCare backed up to the drive when it was identified with a different drive letter or was connected to a different USB port.
If that fails to resolve the problem, 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 ~