Thanks again, Kevin. Yes, as I indicated, I am not using all 4 USB slots simultaneously, and could assign one to a thumb drive. It would mean switching from one slot to another when I switch to my other 'scope rig, but this is just an annoying inconvenience.
I decided to get in touch with my vendor who agreed with me from my description of the problem that the microSD slot was probably faulty. As the unit is still well within its 2-year warranty period, they advised me to return it for repairs. I have decided to do this as I may need to be using all 4 USB ports together some day, and having the microSD slot working would allow me greater flexibility. I'm going to be out of a.p. commission for some weeks (groan), but I deem the result worth it.
If anything of note turns up in this case, I'll add another posting to this thread. But for now, it's going to be in the hands of my vendor/ZWO.
Paul.