windows upgrade : Error creating the Setup Platform key. Error: 0x80070005

Update 1709 to 1803.
Download success, preparations success, installation failure at ~73%

Setupdiag
Error: SetupDiag reports down-level failure found.
Failure Data: Last Setup Phase = Downlevel
Last Setup Operation = Save boot entries for the old OS
Error = 0x80070005-0x5000F

LogEntry: 2019-01-01 22:13:16, Error SP
CSetupPlatform::SuspendAndRelease: Error creating the Setup Platform key. Error: 0x80070005

These are relevant information:

• 0x8 is a Win32 error
• 7 is the device
• 0005 is Access denied
• 0x5 is SP_EXECUTION_UNINSTALL
• 0F is SP_EXECUTION_OP_ADD_UNATTEND

antything else in setupact looks normal up to this entry.

It looks that my system split/migration from one SSD to a new SSD left an EFI partition on the source, now secondary disk (D). This disk contains all libraries like My Documents. Updates failed ever since I have added that drive. I have added an M.2 drive for all system files using a data migration tool by Samsung

I have already tried to

• Reset all registry, file and service permissions ( in safe mode)
• DISM & SFC (no errors)
• restart all services related to the update process
• removed F-Secure & malwarebytes. only standard firewall enabled
• unmount the secondary drive & retry - (getting another error)
• Rename/remove/recreate Catroot & download folders
• browse though all the log files I could find in Panther folders
• Tried to repair with Media Creation Tool for 1803/1809 - same failure
• Prioritize boot from the correct SSD

Need advice if remains of the EFI on the secondary disk indeed is the root-of-all-evil (as suggested in the resolution procedures for windows upgrade), and advice on follow up, i.e. how to check & "safely" remove this EFI partition from my secondary (D-)drive. My C-drive contains all windows system files. MS Technet, Dr Bing & Dr Google don't return anything useful. I need expert advice.

it's an access error after saving the boot entries.

I'd like to know where to look or to find out what this procedure is doing. Technet does not give me a conclusive answer