“cleaning, reinitializing and reformatting did not succeed in making it USB 3.0 compatible.”
In that those things have nothing to do with USB 3.0 not surprising.
"I’ve been working with computers for 25 years and have seen plenty of strange things, but this is right up there with the strangest.
Any useful suggestions are appreciated."
It’s possible your power has encountered a blip effecting each of the pcs. It was 120v now it’s 110v. It’s possible there was a power surge which impacted the pc’s power supplies. It’s possible the vendor of your pcs issued a USB update. It’s possible Microsoft issued a USB update. It’s possible smoke, soot, dust oxidized the USB 3.0 contacts on the pcs USB 3.0 ports. (I actually have an old pc where once a year or so I have to clean it’s USB 2.0 port with emory paper to get it to work again,)
