Did you ever back up using OneCare succesfully? If so, did you change the port that the drive was plugged into or change the drive from when you last did a successful backup? If so, reset your backup plan per the instructions in this post:
http://social.microsoft.com/Forums/en-US/onecarebackupandrestore/thread/5f18a60d-add4-4ff2-87cf-021db7e81917If you have never backed up successfully with OneCare and configuring the backup sees the drive, but running backup after you have saved the plan with that drive as the destination, there may be a conflict with other software that is "holding onto" the drive. Your best bet in this case is to contact support for assistence in determining what might be conflicting or if there is some other reason for the failure.
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
