I know how frustrating it is when you are following a thread and there is no answer at the end of it, so for completeness I can offer the following:

I engaged Microsoft support to help with this drive issue. A tech connected remotely and proceeded to trash my Windows 10 partition to the extent that I had to carry out a full HP Recovery. When I finally got the machine back up and running, and fully restored from backup, the external drive now behaves perfectly. I haven't done anything extra with the drive, yet when the external was reconnected to the recovered Windows, there was the drive letter smiling back at me with full access. I don't propose that anyone trashes their Windows in order to fix a denied access drive problem, but this all points directly to something screwed up within Windows (going back to at least Win 7!), all I can suggest is to try, try and try again, sooner or later, maybe after numerous reboots etc, perhaps with drive connected and not connected, it will suddenly work as per design.