Retrospect Cancel

I'm running Retrospect v. 7.6.123 on a Windows 2003 server.  One of my Backup Sets (on external USB HD) is corrupted and I would like to repair it or erase it and put it on a new drive.  However, in trying to repair the Backup Set on Saturday, I told Retrospect to groom the drive.  Every time I try to start Retrospect (even if I manage to click 'Stop Execution') it tells me that it cannot find media and gives me an explorer dialogue.  All attempts to cancel execution or cancel finding the member for the Backup Set ends in locking up Retrospect.  The only way to stop Retrospect is to shut it down by using the Windows Task Manager.  This results in the grooming process starting over when I restart Retrospect and the lockup again.  When it's locked up, Retrospect works, but all executions attempted result in an error that states that Retrospect is busy with another execution.  I also tried leaving it after I cancel for two hours, and it was right where it was.

How do I cancel this execution and replace the disk in the backup set?

Thank you,

Bill