Spoiler: It’s down to Windows Driver signature verification.
I’ve seen this error message twice:
- Trying to upgrade Windows 8.1 64-bit to the last Windows 10 Insider Build
- Trying to upgrade the Insider Build to the full release of Windows 10 64-bit
The Setupact.log file (Found under C:\$Windows.~BT\Sources\Panther), showed errors when attempting to mount C:\$Windows.~BT\Sources\SafeOS\WINRE.WIM.
Attempting to mount this WIM file manually using DISM, presented another error:
Windows cannot verify the digital signature for this file. A recent hardware or software change might have installed a file that is signed incorrectly or damaged, or that might be malicious software from an unknown source.
As a test I disabled ‘Driver Signature Verification’ using bcdedit from an elevated command prompt:
bcdedit /set testsigning on
After rebooting I tested mounting the WINRE.WIM file again – Success!
And the Windows 10 upgrade? – Worked perfectly.
Don’t forget to run: “bcdedit /set testsigning off” after the upgrade has completed.