It sounds like your backup location may have changed from when you originally configured it.
Try the following to reset the backup plan:
Open OneCare
Click Change Settings
backup tab
Click Configure Backup
Click change settings in the lower left
Make sure that the first screen for Centralized Backup has "No Centralized backup" selected
Click your PC on the left.
Uncheck the selection for "Turn on backup for this PC."
Click Next and save the revised plan.
Go back to the backup tab, configure backup and click Change Settings in the lower left once again.
Select your PC and click.
Click Turn on backup
Select the change location link and select the drive you want to use for backup
Modify the schedule as desired for "When"
Select the file types as desired for "What"
Click next and save the revised plan with the new selection.
Note that your old backups will be okay for a restore, but they cannot be added to and you will need to perform a new, full backup first.
-steve
