https://bugzilla.kernel.org/show_bug.cgi?id=209373
--- Comment #3 from Michel Dänzer (michel@daenzer.net) --- Yeah, the BIOS seems very picky about which partitions it boots from. With GParted or a similar tool, check that the partition which contains the EFI directory has at least the boot & esp flags set. (I only realized this might be enough after manually extracting the BIOS image to a USB HD, and updating from that)