My 2011 MacBookAir boot screen does not list MBR USB flash drives anymore – Why, and how to list them again?
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/sdxn command (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.