Re: UEFI/"BIOS" booting, was Re: USB Install Fails, Complains about CD-ROM
On Sun 13 May 2018 at 19:08:48 (+0200), Pascal Hambourg wrote:
> 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
> > "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 ;
In the case of GPT, I assume the partition entry with the boot flag set
is the protective MBR.
> - 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?
> > "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
Yes, though I assume few people install Windows. It's more likely to
> > "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
"convenient dual boot with GRUB" moves the goalposts.
> >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.
It's very convenient for me. It means I haven't had to interfere with
the way windows chooses to boot, or its configuration of the disk, at all.
> >Switching over involves going through the "BIOS Setup", reached
> >by a separate button (almost recessed).
> As expected.
Not by the author, who would have me reformat the disk as MBR and then
install windows…from where exactly?