Ghost 12 won't release disk space

I have successfully used Ghost for years on my Vista machine, backing up to an external 500GB HD. I have often used the Manage Backup Destination tool to manually delete Recevery point sets whenever disk space became an issue.

 

This time, however, after deleteing a Recovery set, the disk space was not recovered. Ghost shows one set with 243 GB but both Ghost and Windows show the disk with 424 GB used. The space that was used by the older Recovery set has not been released. 

 

Nothing else is on this disk aside from the Norton Bakcups folder. I have run check disk, defrag, and rebooted both the computer and the drive.

 

Any ideas?