Put the 'defective' 970 Pro in (disconnected 850 Evo) and did a clean install from DVD of 1809. In reboot stage that gave a CRITICAL PROCESS DIED error, rebooted a few times and same. So disconnected everything except DVD, kb & mouse. Yes the 840Pro and HGST 4Tb too. Now it went through to install and upgraded fine to 1903. So I have a clean cooked current version 'to go', may yet be using it!

So pulled out the 970 and plugged the 850 Evo and the HGST (because of the relocated Documents etc folder) and had a crack at upgrade. Failed without error again so ran back round and uninstalled Samsung Magician Rapid Drive (which was for the 840 Pro, drive O:). Pretty sure I had it cracked but no, same dang result!

The next 'being stubborn' option is to properly relocate the folders on the mech drive back to the 850 (without moving the files) and pull the mech drive out too, then I have an apples with apples situation for what just worked. Need to do other stuff now, but will get back to that... darn. So close, yet so far...