Is the drive shown as G:\ in Windows Explorer? When was the last time you successfully backed up with OneCare to the drive?
Sometimes, when you unplug the drive and then plug it back in, Wndows will assign it a new drive letter.
At this point, I suggest resetting the backup plan after confirming that you can access the external drive.
http://social.microsoft.com/Forums/en-US/onecarebackupandrestore/thread/5f18a60d-add4-4ff2-87cf-021db7e81917Once you have reset the plan and performed a new full backup, you can delete the older backup, if desired.
http://social.microsoft.com/Forums/en-US/onecarebackupandrestore/thread/f0281206-ed0b-474c-8541-cc50843e5be2If you need help, 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 ~