- The X220 cannot/will not boot GPT disks using Legacy BIOS. I would call this a bug.
- The X220 will not boot /efi/*/*.efi unless "signed"(?) into BIOS, you have to copy it to /efi/boot/bootx64.efi. I guess this is a security feature, but I'm not sure how secure it is if you can bypass it so easily.
- Disabling the BIOS setting "USB UEFI BIOS Support" disables *all*
USB booting, ie, both UEFI and legacy BIOS. If this isn't a bug then
- it at least needs relabelling,
- could be moved/linked to the boot menu and
- is redundant as you can disable USB booting using the Boot menu.
I recommend following these instructions to create a very useful and versatile boot disk for diagnosis.
If you want to use the X220 UEFI boot menu I recommend disabling booting from LAN as it adds a few seconds to the boot time. You have to disable it in several places in the BIOS to avoid the wait.
See also my more general post on Arch Linux on the X220.