I think I said earlier, it's possible whatever PDF handling software you have is "hooking" it.
Or maybe your backup software is doing the same thing.
Without my having to read everything posted again did you try this is Safe Mode?
I had a remote access session with a MS 3rd-line support tech. He just wanted to verify what the issue was.
However, his parting words were along the lines of
"As I removed all the partitions from my hard drive when clean installing Windows v1703, everything (including security permissions) gets reset to default values."
He has now gone away to check what these default values are in 1703.
This would explain why 1703 worked OK after updating my 1607 system 'in situ' - the security permissions would not have changed from 1607 so, effectively, the permissions would be OK in both 1607 & 1703.
I have another remote access session booked for Wednesday, 28th June.
I've been think 'outside of the box' and have a possible work-around,
I've created a new partition on my internal hard drive using EaseUS's 'Partition Manager'; all my other partitions have been created using the MS 'Disk Management' tool.
I copied my 'Test' files to this new partition and attempted to shred them: GREAT NEWS, THEY WERE SHREDDED!!
So it appears that the MS tool MAY be the cause of the issue; I've notified the MS 3rd-line support tech of this.
All bets are off!!
There are inconsistencies between the shredding feature McAfee Internet Security and McAfee Total Protection.
Files that can be shredded using Internet Security CANNOT BE SHREDDED using Total Protection.
This has been verified using
McAfee Internet Security v 14.0.R3
McAfee Total Protection v 16.0.1
by myself with the help of a Microsoft 3rd-line support technician.
Issue raised with McAfee Technical Support on ticket #2269608541