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UEFI fallback boot mode

The UEFI specification defines a so called "fallback boot mode" which allows booting an efi file from disks or removable disks (e.g. USB sticks) without a corresponding boot variable existing in NVRAM.

When the UEFI firmware is directed to boot from a disk it first searches for a EFI System partition (on GPT disks) or a FAT32 partition (on MBR disks). Subsequently, if such a partition is found the firmware searches for a file

\efi
    \boot
         \bootx64.efi
(on x86-64 machines), and if found, starts this efi file.