Hi Earl,  I've done as you suggested.  My mistake last time was not to do a complete rebuild of the database.  After it lost contact with the external drive, I let it re-find all of the photo's.  That seems to be what led to the problem with most of the thumbnails being wrong.

I've made the backups, but haven't had to use them yet.  Some of the colored ball icons are still present in the re-built database, but the correct image generates if I click on it, or hover the mouse over it.  All of the thumbs are correct (so far) and do not need refreshing (which is a big improvement over where I was in restoring the collection).

Very much appreciate your advice.

Tom