UPDATE:
Reconnecting, then Moving only 50 or so at a time worked. But on the last 46, just before finishing, I got "A device attached to the system is not functioning," which is the original error message I got, before changing to Keep Originals in Settings. On retrying (without reconnecting), I got "Device is unreachable."
To my earlier complaints, add my frustration with Windows unfathomable error messages. Their obscurity was understandable under DOS and when Windows machines had limited memory. But, hey, MS, this is 2019! Be specific.
Transferring one at a time worked for a couple files, but then required reconnecting yet again. (There has been NO delay between these attempts.) And then worked for only one file. Reconnecting again and again enabled me to copy/move a few files at time, then stopped working altogether.
Obviously there is NOTHING actually wrong with my devices or settings.
Where do I send the bill for an hour of wasted time?