Fix error 0x80070003 when updating Windows 10 1809 to 1903

Errors you may see:

• Operation failed: Mount WIM file[...]
• Error 0x80070003
• 'Windows 10 failed to install.'

If you have never installed Macrium Reflect including in the past, follow another guide, as there are many other possible causes. I had tried everything, and finally came across a mention that the issue stems from Macrium Reflect. This issue only occurs after you uninstall Macrium Reflect!

In regedit, go to HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\WIMMount.

If you find the value of ImagePath to be \??\C:\Program Files\Macrium\Reflect\wimmount.sys, shake your fist at Macrium for literally breaking your system, and change the value back to the default of system32\drivers\wimmount.sys.