Solved Lenovo X220 amd64 UEFI loader

According to

The amd64 UEFI loader has successfully booted on the following devices:
  • Lenovo Thinkpad X220 with UEFI firmware 1.31
I've been trying to boot my X220 using this loader today and keep hitting mountroot> problems. Could this be connected and how do I found which firmware I'm using?
Unfortunately it didn't.... One X220 was at 1.39 but the update to 1.43 made no difference. Trying a different X220 which was at 1.29 was able to boot so it looks like it has nothing to do with the BIOS level.