Be sure the drive has been removed from Time Machine before attempting to erase it.
In this case, I don't think the TM drive would have been an issue as I was replacing the Carbon Copy Cloner drive. However, when drive changes are involved, I have always had a practice of going to "Shutdown" first.
Special Mention:I had wondered about keeping my old Carbon Copy Cloner data and popped a quick note to CCC, advising how I thought I might do it. They were quick to respond with a "We don't advise that way because..." and provided guidance on the right way. Pretty good service.
