Since the drive has nothing else on it other than the backup, your best bet is to format the external drive.
Open Windows Explorer and find the external drive. Right click the drive and select Format. Quick format is fine.
Once you have formatted the drive, reset your backup plan and set up a new, full backup.
Details on how to cleanup backup:
http://social.microsoft.com/Forums/en-US/onecarebackupandrestore/thread/f0281206-ed0b-474c-8541-cc50843e5be2How to reset the backup plan:
http://social.microsoft.com/Forums/en-US/onecarebackupandrestore/thread/5f18a60d-add4-4ff2-87cf-021db7e81917-steve
Microsoft MVP Windows Live / Windows Live OneCare, Live Mesh, & MS Security Essentials Forums Moderator