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Re: [Bug-xorriso] ISO installer image: GPT versus MBR partitions

From: Thomas Schmitt
Subject: Re: [Bug-xorriso] ISO installer image: GPT versus MBR partitions
Date: Wed, 17 Apr 2019 12:23:56 +0200


Florian Pelz wrote:
> $ gdisk Downloads/debian-live-9.8.0-amd64-xfce.iso
> ...
> Found valid MBR and GPT. Which do you want to use?
> 1 - MBR
> 2 - GPT
> ...
> Your answer: 2
> Using GPT and creating fresh protective MBR.
> Warning! Main partition table overlaps the first partition by 64 blocks!
> You will need to delete this partition or resize it in another utility.
> ...
> Number  Start (sector)    End (sector)  Size       Code  Name
>    2            1432            2263   416.0 KiB   0700  ISOHybrid1

This operation has killed the MBR partition table with the entry for
the EFI partition.
Due to the odd layout introduced for Fedora ISOs in SYSLINUX program
"isohybrid", there is a GPT partition entry which marks the EFI FAT image.
But it does not have the EFI type GUID. So EFI won't recognize it.

(This layout has been adopted by Debian and others who use ISOLINUX
 for BIOS and GRUB for EFI. It needs MBR partition type 0x00 for the
 ISO partition. The GPT ISO partition is madly starting at block 0.
 Partition editors are entitled to hate it.
 But well, the user decides what to do. I can only give advise.)

There remains the boot path of legacy BIOS via the MBR x86 code.

> The Debian ISO works fine and is bootable from its EFI partition.

Did you try booting before or after the gdisk run ?

> > Is guixsd-install-0.15.0.i686-linux.iso
> > suitable for that machine (modulo GPT backup position) ?

> Without changing the ISO’s partition tables, it does not work.
> What should I change?

I will develop and test a xorriso repack run for the mounted ISO
which produces MBR partitioning without GPT and APM.
Maybe this evening.

Have a nice day :)


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