Hi, Nicolas George wrote: > 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.
That's what the UEFI specs prescribe. GPT is defined by UEFI-2.8 in chapter 5 "GUID Partition Table (GPT) Disk Layout". Especially: 5.2.3 Protective MBR For a bootable disk, a Protective MBR must be located at LBA 0 (i.e., the first logical block) of the disk if it is using the GPT disk layout. The Protective MBR precedes the GUID Partition Table Header to maintain compatibility with existing tools that do not understand GPT partition structures. > 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. Given the creativity of firmware programmers, this is not impossible. At least the programmers of older versions of OVMF took the presence of a GPT header as reason to boot, whereas without it did not boot. Meanwhile this demand for GPT debris has vanished and OVMF boots from media with only MBR partitions, too. I wrote: > > This layout [used by Debian installation ISOs] was invented by Matthew > > J. Garrett for Fedora > I think I invented independently something similar. > https://www.normalesup.org/~george/comp/live_iso_usb/grub_hybrid.html Not to forget Vladimir Serbinenko who specified how a grub-mkrescue ISO shall present its lures for BIOS and EFI on optical media and USB stick. The ISO has a pure GPT partition table, where the ISO filesystem is not mountable as partition but only by the base device (like /dev/sdc) or possibly by its HFS+ directory tree via the Apple Partition Map, if present. (To create such an ISO, install grub-common, grub-efi-amd64, grub-efi-ia32, and grub-pc. Then run grub-mkrescue with some dummy directory as payload. The ISO will boot to a GRUB prompt.) Have a nice day :) Thomas
