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Re: UEFI/"BIOS" booting, was Re: USB Install Fails, Complains about CD-ROM




Le 14/05/2018 à 02:02, David Wright a écrit :
On Sun 13 May 2018 at 19:08:48 (+0200), Pascal Hambourg wrote:

Most of my early experience with UEFI boot comes from a rather old
Intel motherboard. Beside crippled UEFI support (no UEFI boot from
USB or SATA in AHCI mode), it had a couple of annoying requirements :
- boot in legacy mode only if the MBR contains a partition entry
with the boot flag set, regardless of whether the disk has a MSDOS
or GPT partition table. This behaviour is beyond any common BIOS
standard, but I have observed it on many other systems, mostly Dell
and HP ;

In the case of GPT, I assume the partition entry with the boot flag set
is the protective MBR.

Yes, as the protective GPT partition entry is the only non-empty entry.

- boot in EFI mode from GPT only if the protective partition entry
in the MBR has the boot flag unset. I admit this requirement is part
of the GPT specification, but really do not see the point in
enforcing such a minor detail.

Anyway, these two requirements put together make it impossible to
boot in both legacy and EFI mode from the same GPT disk with this
motherboard. However they allow to boot in both modes from the same
MSDOS disk. But who still wants to use MSDOS format nowadays ?

Is it impossible, then, to change the boot flag in a protective MBR?

Of cours not, but it is even less convenient than switching the boot mode in the firmware setup : boot the system, switch the boot flag, reboot the system...