Ok, last update on this thread. I was working for four nights so I didn't want to start anything complicated. Got off from work Thursday morning and started this process again, a little bleary-eyed but with a full load of coffee.
I had thought about what "hardware" might be having an issue. Normally, troubleshooting means "change one thing only, then try, then another thing, try again" etc. Rather than waste time, I just (1) disconnected the TV set I was using as a second screen (cloned via HDMI cable), disconnected the camera used for TrackIR5, pulled the USB cable from the printer (since that has an SD card-reader in it) and finally changed the drive letter for the DVD drive to "Z".
1. Inserted the flash drive in, gets assigned letter E, still has an issue with hardware.
2. Tried again using the ISO file I had copied to the hard drive (so this time there's only Drive C and Z), still has an issue with hardware.
3. Booted from the flash drive and tried again. Still an issue with hardware (???).
Would you like to perform a full install of Windows 10 instead?
Well, I guess I don't have much of a choice at this point, do I?
I'm so glad I have (1) installation disks of everything and (2) full, comprehensive back-ups of all data and everything in the Steam folder.
I did notice a glitch when getting the updated drivers for the video card - had an issue (failed to install) the Nvidia Experience (current version). Restarted that process, this time without the drivers checked off, and it went through ok. Not happy having to spend from 10 am to 1:30 the next morning putting all this back together and I still have some "tweeking" to do (minor adjustments here and there) but at least it's done and over with. Tested all games in Steam last night, everything works.
It is like giving birth. Thank you, Micro$oft, for the experience.