Unfortunately, a reinstall of OneCare means that you must now perform a new, full backup. While the old backups are valid, and can be restored from, if a new full backup requires more space, you will need to delete the old backups.
See this post for how to do this - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=1698096&SiteID=2
Of course, if this drive is only used for backups, formatting the drive will be the easiest way to recover the space.
-steve