Thank you for the quick reply. I too have seen the symptom before, but only when another windows computer was running MC and using the library; when a second computer starts MC with the same library, MC sensibly announces that the library will be opened in read-only mode, for obvious reasons. In my case, though, the Id uses its own library (watching the audio only portion of the database), so it can't be the prior copy of MC with the library open. Thus, although I tried your suggestion, it didn't work.
What fixed my problem is what I should have done before I posted my question: I reinstalled MC, restored the last known good library backup, and the problem is gone. This would seem to indicate that MC had set a durable flag that the library was in use and no one should step on it until MC was done modifying it. Just a guess.
I wonder if what induced my problem was that when I answered No when the Rename, Move & Copy Files function asked if I wanted to keep the changes, I didn't wait long enough for MC to unwind the changes I had proposed. After a while nothing much seemed to be happening to the contents of the database directory it was operating on, so I took over and manually restored the directory. I can see how MC could be in a slightly disheveled state if I interrupted it in mid-process. If this is likely what I did wrong, I'm wondering if some kind of progress indicator, or just a small "Working on it" indicator would be a useful enhancement.
Thanks again for your help. This user community is one of the best I've ever encountered.
Stan