I have an Intel NUC, which has been behaving fine up until today. I restored my system from a backup (using fsarchiver) and have set up GRUB using a USB stick and a chroot environment. I have these partitions:

• /dev/sda1: EFI partition (vfat)
• /dev/sda2: /boot (ext2)
• /dev/sda3: LVM (root filesystem on /dev/system/root)

grub-install runs without errors, but there's a warning that "fi variables are not supported on this system"

When I try to boot my system I am dumpted to the EDI prompt. I am able to boot my system from this using
blk0:\> .\EFI\debian\grubx64.efi
But it won't boot automatically. The BIOS only shows "EUFI: Built-in EFI Shell as a boot option under Boot Order -> UEFI. The hard drive isn't listed.

Is this a software or a BIOS problem? I haven't been able to find a BIOS update file (it's a pretty old machine).

I have tried resetting the BIOS to default values with no luck

CSM = enabled
in UEFI is one possible reason for "EFI variables are not supported on this system".
Go to UEFI, disable CSM and try again grub-install.

For some reason, legacy boot only works if I select the disk from the boot menu.

I fixed this, and got rid of the "efi variables are not supported on this system" by issuing this before running grub-install in the chroot environment:

# mount -t efivarfs efivarfs /sys/firmware/efi/efivars