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

[pelzflorian (Florian Pelz)]

On Wed, Apr 24, 2019 at 10:32:08PM +0200, Daniel Kiper wrote:
> 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?
> 

This applies to all Guix images, including
guixsd-install-0.16.0.x86_64-linux.iso,
guixsd-install-0.15.0.i686-linux.iso, and current Guix git master.  It
is created by a grub-mkrescue invocation.  See:

https://git.savannah.gnu.org/cgit/guix.git/tree/gnu/build/vm.scm

Some patches are applied to grub-mkrescue.  See those prefixed with
“grub” at:

https://git.savannah.gnu.org/cgit/guix.git/tree/gnu/packages/patches

For completeness, the GRUB package is defined here to use these patches:

https://git.savannah.gnu.org/cgit/guix.git/tree/gnu/packages/bootloaders.scm



> > 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
> >

Note that more recent Macbooks appear not to be affected (at least an
“early 2014” MacBookAir6,2 is not affected).


> > 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


I am not very familiar with partitioning.  Is any image created by
grub-mkrescue a floppy image?  Creating a disk image without
grub-mkrescue works and boots fine on the USB drive, so do the above
ISO images when burned to a DVD, but the goal is that users can dd the
ISO to a USB drive and it just works.

Regards,
Florian