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

Le 13/05/2018 à 17:18, David Wright a écrit :
On Fri 11 May 2018 at 15:13:04 (-0500), Kent West wrote:

That's good to know. I guess my source material (
is wrong. Or I misunderstood it.

While a lot of the detail on that long page might be correct, there
are also statements there that don't seem to agree with reality.

Most of the statements you quoted agree with my (admittedly limited) experience with UEFI. There is a difference between the theory (specifications) and the reality (implementations), and some pieces of software may have extra requirements beyond the sole UEFI specification.

     "I really can’t recommend strongly enough that you do not attempt
     to mix UEFI-native and BIOS-compatible booting of
     permanently-installed operating systems on the same computer, and
     especially not on the same disk. It is a terrible terrible idea
     and will cause you heartache and pain. If you decide to do it,
     don’t come crying to me." (under "UEFI booting: background").

I would not be as much adamant as the author, but my experience says : it can work, but expect trouble.

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 ; - 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 ?

     "Disk formats (MBR vs. GPT)

       Here’s another very important consideration:

       If you want to do a ‘BIOS compatibility’ type installation, you
     probably want to install to an MBR formatted disk. If you want to
     do a UEFI native installation, you probably want to install to a
     GPT formatted disk."

I do not agree so much with this one when it comes to install GNU/Linux. But it is an absolute requirement when installing Windows.

     "A specific example

     To boil down the above: if you bought a Windows 8 or later system,
     you almost certainly have a UEFI native install of Windows to a
     GPT-formatted disk. This means that if you want to install another
     OS alongside that Windows install, you almost certainly want to do
     a UEFI-native installation of your other OS. If you don’t like all
     this UEFI nonsense and want to go back to the good old world
     you’re familiar with, you will, I’m afraid, have to blow away the
     UEFI-native Windows installation, and it would be a good idea to
     reformat the disk to MBR."

I agree with the author. If you want to keep the existing EFI Windows installation and have a convenient dual boot with GRUB, you'll have to set up your favourite distribution to boot in EFI mode. If you want to go back to legacy boot, including for Windows, you'll have to repartition the disk to MSDOS format and reinstall Windows.

I can't reconcile that with the system here, a Windows 8→10 UEFI laptop
and GPT disk running linux in BIOS compatibility mode (here called
Legacy mode by Lenovo) booting from an MBR on an ATA disk:

That is not very convenient, is it ? You cannot boot Windows boot manager from GRUB nor you can boot GRUB from Windows boot manager and must select the boot mode in the UEFI firmware setup whenever you want to switch the operating system.

Switching over involves going through the "BIOS Setup", reached
by a separate button (almost recessed).

As expected.

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