Re: grub-mkrescue: Problem with MBR partition table at start of EFI partition

[Daniel Kiper]

On Sun, Apr 21, 2019 at 03:43:52PM +0200, Thomas Schmitt wrote:
> Hi,
>
> this is about how exactly to solve a diagnosed and worked-around problem.
>
> Guix is one of the few distros which make their installation ISOs by
> help of grub-mkrescue. The EFI boot manager of an old Macbook got stuck
> when such an ISO was presented on USB stick. I.e. it not only did not
> boot the ISO but showed no other EFI partitions either.
>
> A Debian Live 9 ISO does not impose such a problem.
> The same Guix ISO on DVD rather than USB stick works well, too.

How do you create USB stick from ISO image? Have you tried latest Guix image?

> The owner of the machine is Florian Pelz (Cc'ed). He characterizes it as:
>   MacBook Pro (13-inch, Mid 2010)
> MacOS Yosemite reports:
>   Model Name:            MacBook Pro
>   Model Identifier:      MacBookPro7,1
>   Boot ROM Version:      MBP71.003F.B00
>   SMC Version (system):  1.62f7
>
> During a longish bug hunt it turned out that the decisive difference
> is the first block of the FAT filesystem image which Debian creates by
> mkfs.fat whereas grub-mkrescue creates it by mformat.
>
> The mformat-made image has an MBR partition table entry which claims the
> whole image as partition 1:
>
>   Device            Boot Start   End Sectors  Size Id Type
>   /mnt/iso/efi.img1 *        0  2879    2880  1.4M  1 FAT12
>
> If this partition entry is zeroed, then the EFI boot manager works even
> when the USB stick with this modified ISO is plugged in.

Numbers here and in another email suggest that this is a floppy image.
And floppy images do not have MBR. They have something called boot
sector which is a bit different thing. I have a feeling that simply Macs
tested by you does not like floppy images. If you create regular HDD
images then probably everything will work.

Daniel