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




On Mon 14 May 2018 at 23:29:43 (+0200), Pascal Hambourg wrote:
> 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...

Sorry, I thought impossible meant not possible. I agree that booting
up a system (that happens to be set to the wrong state) just to change
the boot flag and then reboot is a lot less convenient than the
process I use (which always lets you boot into the correct system
first time, and only requires action when you want to change over).

But the fact that you've experienced a crippled mobo does not IMHO
mean that some of the scenarios you could use on less crippled systems
shouldn't be discussed—nay, be treated as near impossible—in a web
page that's meant to be throwing light on the matter. As written, the
author just pours special magic sauce around, whatever that is.

Cheers,
David.