Answering TomH: I copied my *.rmgc db file (that is in my Documents directory) to my Music directory (because my Google Drive does not backup music). I then ran RootsMagic from the db file in my Music directory. Backup works perfectly! I do not get that error message! This certainly appears to prove that Google Drive is the culprit. I can live with this arrangement.
Thanks so much for your insightful suggestion!