> that is what Thomas meant with hybrid ISO having both, good old MBR
> BIOS and EFI support.

That's modern extra hybrid: {PC-BIOS, EFI} x {DVD, USB-stick}
The older ISO hybrid concept is {PC-BIOS} x {DVD, USB-stick}

For the old hybrid, T2 uses a GRUB x86 El Torito Image and a GRUB MBR which
is specialized on executing the El Torito Image as x86 program.
This boot equipment can be seen in
The El Torito image is accessible in the ISO as

The newer extra hybrids additionally have an EFI System Partition image as
file (Fedora, Debian amd64, ...) or as nameless appended data blob (Knoppix
This image is a FAT filesystem with the UEFI prescribed start programs
and some GRUB menu (Knoppix uses SYSLINUX which does not work from DVD).
The image gets advertised in the ISO as El Torito image for EFI and as
partition of MBR type EF or of GPT Type GUID

As said, UEFI prescribes these partition IDs for hard-disk-like devices and
El Torito for optical media. Many EFI implementations look at more places
for the /EFI/BOOT/BOOT*.EFI programs. Microsoft Inc. obviously expects that
all FAT filesystems get inspected which have a Microsoft MBR partition type
for data. E.g. 0C or 0E. There are howtos which rely on this. No firmware
implementer will dare to insist in a narrow interpretattion of UEFI specs.

The fact that an inofficial location of the EFI programs works on one
firmware does not indicate that it would work on the next firmware.
So it is better to obey the specs veven if they are fewly enforced.

Have a nice day :)


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