OK, I did a test and YES, the Boot.WIM file works and using GImageX works fine to install the required files for windows.
However... I then try to run BCDBoot.exe from that external drive and I come up with an error:
The program can't start because api-ms-win-core-heap-I2-1-0.dll is missing from your computer. Try reinstalling the program to fix this problem.
I am trying to make a Windows 10 external drive, from a machine that is currently running Windows 7. Does that make a difference? Any other reasons why that file isn't present? Could it be that Boot.WIM doesn't have that file and should?