Thomas Schmitt (12024-01-24):
> The Debian installation and live ISOs have MBR partitions with only a
> flimsy echo of GPT. There is a GPT header block and an entries array.
> But it does not get announced by a Protective MBR. Rather they have two
> partitions of which one is meant to be invisible to EFI ("Empty") and
> one is advertised as EFI partition:
>
> $ /sbin/fdisk -l debian-12.2.0-amd64-netinst.iso
> ...
> Disklabel type: dos
> ...
> Device Boot Start End Sectors Size Id Type
> debian-12.2.0-amd64-netinst.iso1 * 0 1286143 1286144 628M 0 Empty
> debian-12.2.0-amd64-netinst.iso2 4476 23451 18976 9.3M ef EFI (FAT-12
>
> So any system which boots this ISO from USB stick does not rely on
> the presence of a valid GPT.
You seem to be assuming that the system will first check sector 0 to
parse the MBR and then, if the MBR declares a GPT sector try to use the
GPT.
I think it is the other way around on modern systems: it will first
check sector 1 for a GPT header, and only if it fails check sector 0. Or
not check sector 0 at all if legacy mode has been removed.
> This layout was invented by Matthew J. Garrett for Fedora and is still
> the most bootable of all possible weird ways to present boot stuff for
> legacy BIOS and EFI on USB stick in the same image.
I think I invented independently something similar.
https://nsup.org/~george/comp/live_iso_usb/grub_hybrid.html
Regards,
--
Nicolas George