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Re: Unable to boot BIOS/GPT system.

Le 23/11/2018 à 09:11, Thomas Schmitt a écrit :

Pascal Hambourg wrote:
If the EFI firmware can boot in legacy BIOS compatibility mode, it may
require to set the boot flag on the protective GPT partition entry in the
protective MBR.

According to user reports on grub-devel mailing list about grub-mkrescue
ISOs, the boot flag must not be set on the protective MBR partition.

Yes, the protective partition should have the boot flag cleared for compliance with the GPT specification. But that's the theory, and there is the practice. For example I have come across an UEFI firmware implementation on a very old Intel board which required the boot flag to be cleared for EFI boot but set for legacy BIOS boot...

But some BIOS implementations wont't regard the medium as bootable if there
is no boot flag set at all.

IME, many Dell and HP's are among these.

The compromise that finally worked for those who tested, was to create an
MBR partition of type 0x00 starting at LBA 0, having only one block, and
bearing the boot flag.

Indeed it worked for me too, and even allowed booting either in EFI or BIOS mode with the above board. At least until parted (or, I guess, any other libparted-based partition manager such as partman, the partitioning program part of the Debian installer) is used on the disk and resets the partition table to a standard protective MBR. Same if you set up a hybrid MBR with gdisk. So make sure to never use parted & Co. on such a disk or make sure you set the boot flag again afterwards.