What is your backup device?
Did you use this device for OneCare backup before or are you changing to a new backup location/configuring backup for the first time?
If you are changing the location, see this post:
http://social.microsoft.com/Forums/en-US/onecarebackupandrestore/thread/5f18a60d-add4-4ff2-87cf-021db7e81917If you are not changing the location, but OneCare is no longer seeing the drive, it may have changed drive letters due to be unplugged and then plugged into a different port. If so, the link above will resolve it, but you will be starting a new backup set.
If you are setting up backup for the first time, note that OneCare can backup to CD/DVD, External USB or a network share. It cannot backup to an eSATA drive or internal drives.
If you need help with this, you can 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 ~