Re: [Bug-xorriso] grub-mkrescue does not boot on older Macs as USB stick

[Alexander E. Patrakov]

23.12.2015 23:44, Andrei Borzenkov пишет:
> OK, it turned out to be not related to xorriso, but for the record ...
>
> Mac does not like FAT created by mformat. I can reproduce it easily by
> creating single MSDOS partition on USB stick of type 0xef size 2880K,
> formatting it with same options as grub-mkrescue:
>
> sudo mformat -C -f 2880 -L 16 -i /dev/sdb1 ::
>
> and copying bootx64.efi into \EFI\BOOT on it.
>
> The same partition formatted with mkfs -t fat works like a charm.
>
> Both Porteus and openSUSE CD (which correctly boot) have ESP created by
> mkfs.fat.
>
> TBH I think that precreating FAT once and shipping image with GRUB is
> the most simple solution. It can be empty, we can dd files later.
>
> Ideas?

I have byte-compared the boot sectors (because that's the only different 
things in the filesystems).

So, here are the differences:

Bytes 0x03 .. 0x0a represent the OEM ID (software that created the 
filesystem). mformat: "MTOO4018", mkfs.vfat: "mkfs.fat". Should not be 
significant.

Bytes 0x11 .. 0x12: a WORD that represents the number of root directory 
entries. mformat: 240, mkfs.vfat: 224. This is settable via the -r 
parameter of mformat.

Bytes 0x27 .. 0x2a: a DWORD with the serial number of the filesystem. 
Should not be significant.

Bytes 0x3e .. 0x1fe: x86 bootstrap code, which is not executed and thus 
is likely not significant.

If you make the number of root directory entries match (using -r 14), 
then there appears a difference in bytes 0x16 .. 0x17, which form a WORD 
that means the number of sectors per FAT. mformat -r 14: 16, mkfs.vfat: 
9. But this is settable with -L.

So, could you please run this?

sudo mformat -C -f 2880 -L 9 -r 14 -i /dev/sdb1 ::

If this fails, try giving mformat a known-good x86 boot sector using the 
-B option.

--
Alexander E. Patrakov