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The EFI implementation of some older MacBook laptops stalls when it gets presented a grub-mkrescue ISO image for x86_64-efi target on an USB stick. Affected are models of year 2010 or earlier. Workaround is to zeroize the bytes 446 to 461 of the EFI partition, where mformat has put a partition table entry which claims partition start at block 0. This change will not hamper bootability on other machines.