Thanks for the info that this is a KNOWN bug in Windows 10 and your suggestion to use copy instead. Unfortunately, I never heard about it until I got burned (isn't that how it always happens).
Did this creep in in a recent update or has it been lurking waiting to pounce for a while?
Using copy is not always very practical, so I hope MS is actively trying to fix this. IMHO, this is a major bug because it is such a commonly used command and can have disastrous effects.
I say copy is not always practical, because this particular directory had a large number of pictures, so it takes a long time to copy. After I restored the directory from an old backup and found (all I think) the files that were not in the backup, but we're on another machine, it took a couple of hours to do the 2 tree copies required to do this without using move.
BTW, I did try a recovery utility but it couldn't find what Windows had done with them. I probably waited too long to use it.
Again thank you for your response. I was lucky that I did have the data available in other locations.