I have a MBR-formatted, bootable USB flash drive with GRUB. I have tested it successfully on other computers – one with BIOS and another one with UEFI.
I usually boot my 2011 4,1 MacBookAir with that flash drive. The last time I did that was 21 October.
Yesterday, I restarted, pressed Option after the gray screen as usual, but I only saw "Macintosh HD" and "Recovery" – I do not see the flash drive, which is typically labeled "Windows".
I zeroed the key and reinstalled the bootloader, but that did not fix the issue. However, when I connect a GPT-formatted disk, the boot screen shows it with the label "Windows", as it normally does.
It would be helpful to know whether other people can (or cannot) reproduce the issue on the same model.
Best Answer
Solved - if no partition of the disk has the bootable flag, the disk does not appear at the boot screen.
In my tests, if the partition that has the bootable flag has been formatted with
mkfs.vfat /dev/sdxncommand (without options) using mkfs.vfat versions 3.0.14 or 3.0.17, the disk appears at boot, but, when the disk is selected, the bootloader is not run. Instead, the message "This is not a bootable disk" is printed.The partition that is flagged as bootable does not have to be the one that contains the bootloader configuration files, and it may have MBR partition type 83 and/or ext4 or btrfs filesystem.