I had the same problem with that error code. For me, it was a driver conflict, and my system image backup was enough to restore everything when I did a clean install instead.
You could also try running the Windows Update Troubleshooter first, as it can sometimes automatically fix issues blocking the upgrade. Let me know if that works.
I had the same problem with that error. For peace of mind, I manually copied my Documents and Pictures folders to that external drive as well, just in case.