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




On Mon 14 May 2018 at 09:14:23 (-0400), Stefan Monnier wrote:
> > That said, there are other statements that are odd:
> 
> Not sure what you find odd about them:
> 
> >     "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").
> 
> Here he's just saying that there's a good chance you'll encounter
> difficulties if you try that, and indeed in my experience the behavior
> of such a config will depend on undocumented details.

Yes, documentation of firmware is almost unknown in my experience
(since probably 30 years ago). That's why I took the least invasive
method that I could. Using a UEFI/UEFI approach means that you have
to explore the manufacturer's undocumented implementation of UEFI.
Plenty of horror stories there, including Pascal's.

> >     "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."
> 
> Same here (basically for the same reason: the behavior of the firmware
> and OS when faced with a disk that has both a GPT and an MBR partitions
> is largely unspecified and will vary depending on your system).

Eh? I've yet to see a GPT disk that didn't have a protective MBR.
I thought that's the reason why GPT starts at block 1 and not
block 0: an MBR was designed into GPT from the start (no pun intended).

> > 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:
> 
> It's easy to reconcile: he doesn't say your setup is impossible or can't
> work, he just recommends not to do that because you may encounter
> unexpected difficulties.  E.g. in theory an upgrade to your firmware or
> to one of your OSes could break it, tho in practice you're probably OK
> at least until you move that setup to another machine with
> a different firmware.

Not sure what you mean here. It's a laptop: nowt's going nowhere.

But in a page as long as this one is, I think the author is rather
dismissive of using Legacy mode at all. Perhaps the clue is here:

    "Don’t do UEFI-native installs to MBR-formatted disks, or BIOS
    compatibility installs to GPT-formatted disks (an exception to the
    latter is if your disk is, IIRC, 2.2+TB in size, because the MBR
    format can’t handle disks that big – if you want to do a BIOS
    compatibility install to a disk that big, you’re kinda stuck with
    the BIOS+GPT combination, which works but is a bit wonky and
    involves the infamous ‘BIOS Boot partition’ you may recall from
    Fedora 17).

I haven't been able to find anything infamous about the BIOS Boot
partition but it sounds as if the author had a bad experience at
sometime in the past which has affected their ability to view the
topic objectively.

Cheers,
David.