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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: Tue, 16 Apr 2019 23:15:56 +0200


i wrote:
> > A broken backup GPT is not supposed to stop EFI from finding its partition

Florian Pelz wrote:
> On the 2011 Macbook I am using, the boot process gets stuck before I
> can select the drive to boot from, as long as a USB flash drive with
> this GPT error is in a USB port.

Did you already test whether it works after you treated the stick
with gdisk expert option 'x' 'e' ?

If it really does work, then i am glad to have written "supposed".

It would then be interesting to see whether this picky machine boots
from a MBR partition table.
suitable for that machine (modulo GPT backup position) ?
If so, i will use my copy of this ISO to develop a xorriso run on
the mounted ISO which produces a new ISO with MBR-only layout.

(next mail from Florian):
> Actually it still gets stuck after I used fdisk to repair the drive’s
> GPT.  I will check again with a Debian ISO tomorrow.

Did the GPT repair really work ?
I.e. do other partition editors or kernels have no complaints about the
stick any more ?

Another try would be to deface the Apple Partition Map. It's a Mac.
So APM and HFS+ might have some impact. But other than expected by

With APM the first bytes of the ISO are
  45  52  08  00  eb  02  ff  ff
(which is the APM magic number and also x86 code without effect.)

Without APM they are other intentionally effectless x86 code:
  eb  63  90  90  90  90  90  90

If you write the latter 8 bytes over the start of your USB stick, then
APM should not be recognizable any more.

> I will check again with a Debian ISO tomorrow.

Debian installation ISOs have APM but no HFS+. I.e. their APM is quite
useless. These are: "netinst" ISOs, and the ISO sets for CD, DVD, BD, BDDL.

Debian 9 Live ISOs have no APM (and no HFS+).
Version 8 has no EFI equipment but only for legacy BIOS.

Have a nice day :)


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