You never mentioned it was on a laptop. If it is try another USB. On a laptop you have USBs that are controlled via a 3rd party driver like Marvel and then you have ones controlled by the chipset either Intel or AMD. Generally only one of them is controlled by the chipset. Try a different USB. File backups can be finicky with third party hardware.
What do you mean with third party hard ware? A third party drive...? (Surely everyone uses third party drives?)
I don't understand why different USB ports should make a difference...
What has happened though : the drive that I used (and that file history is looking for OS:(G)) has (somehow) changed to OS:(H)
I don't really understand how that's happened.