Sorry. It was two tries. The first kept the USB in all the way.
I don't have two internal HDD's. I'd have to run the .iso from the only one I've got, or from an external USB HDD. Hopefully the external HDD wouldn't suffer the same fate as an external flash/thumb drive.
I do not leave any USB drive plugged in all the time. But I do have this...
It only shows up when "show hidden devices" is selected. It hasn't showed up normally since I left Win 8.1 for the original Win 10. It's got a socket on the right side of this Dell Inspiron 3542, but I never plugged anything into it. So I don't miss it.
I wish Setup.exe would actually name the particular USB drive that makes it disagreeable.