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

On Fri 11 May 2018 at 15:13:04 (-0500), Kent West wrote:
> On Fri, May 11, 2018 at 2:59 PM, Pascal Hambourg <pascal@xxxxxxxxxxxxxxx>
> wrote:
> > Le 11/05/2018 à 20:33, Kent West a écrit :
> >
> >>
> >> I learned that EFI boot drives need to have a GPT partition table. On a
> >>
> >
> > This is not correct. The UEFI specification supports boot from a drive
> > with an MSDOS partition table. Otherwise why would there be an "EFI system
> > partition" type identifier (0xef) for MSDOS partition tables ? The Debian
> > installer hybrid image has an MSDOS partition table. (It also has Apple and
> > GPT partition tables, but they are bogus)
> That's good to know. I guess my source material (
> https://www.happyassassin.net/2014/01/25/uefi-boot-how-does-that-actually-work-then/)
> 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.

It does make a statement that agrees with Pascal's:

    "and the UEFI spec requires that UEFI-compliant firmwares be
    capable of interpreting GPT (it also requires them to be capable
    of interpreting MBR, for backwards compatibility)." (under "The
    GPT (GUID partition table) format").

That said, there are other statements that are odd:

    "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").
    "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."
    "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 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:

# gdisk -l /dev/sda
GPT fdisk (gdisk) version 1.0.1

Partition table scan:
  MBR: protective
  BSD: not present
  APM: not present
  GPT: present

Found valid GPT with protective MBR; using GPT.
Disk /dev/sda: 976773168 sectors, 465.8 GiB
Logical sector size: 512 bytes
Disk identifier (GUID): 2653F13F-2CD0-406F-B004-6AFFDBE18127
Partition table holds up to 128 entries
First usable sector is 34, last usable sector is 976773134
Partitions will be aligned on 2048-sector boundaries
Total free space is 4077 sectors (2.0 MiB)

Number  Start (sector)    End (sector)  Size       Code  Name
   1            2048         2050047   1000.0 MiB  2700  Basic data partition
   2         2050048         2582527   260.0 MiB   EF00  EFI system partition
   3         2582528         4630527   1000.0 MiB  ED01  Basic data partition
   4         4630528         4892671   128.0 MiB   0C01  Microsoft reserved ...
   5         4892672       347348991   163.3 GiB   0700  Basic data partition
   6       347348992       429268991   39.1 GiB    8300  Linux-A
   7       429268992       511188991   39.1 GiB    8300  Linux-B
   8       511188992       883275775   177.4 GiB   8300  Linux-Home
   9       883275776       883292159   8.0 MiB     EF02  Linux-BIOS-Boot
  10       883292160       892084223   4.2 GiB     8200  Linux-Swap
  11       892086272       892803071   350.0 MiB   2700  
  12       892803072       894900223   1024.0 MiB  0700  Basic data partition
  13       894900224       947329023   25.0 GiB    0700  Basic data partition
  14       947329024       976773119   14.0 GiB    2700  Basic data partition

 => Grub2 (v1.99) is installed in the MBR of /dev/sda and looks at sector 
    883275776 of the same hard drive for core.img. core.img is at this 
    location and looks in partition 85 for . [from bootinfoscript].

The original sda5 was shrunk, and five new partitions added, all by
windows. Then the machine was booted in compatibility mode and linux
was installed. Finally, I told windows not to assign a drive letter
to the linux partitions (so that it does not keep offering to reformat
them at every encounter).

The result is a laptop that boots windows in UEFI Boot Mode and
linux in Legacy Mode. Windows knows nothing about the linux
installation except to ignore those partitions. Linux mounts
what it can of the windows installation, but in readonly mode.
Switching over involves going through the "BIOS Setup", reached
by a separate button (almost recessed).

All this makes me treat the information on that page with suspicion.
Perhaps the statement quoted here is self-referential (but lower than
95%, I hope): "You’ve probably read a lot of stuff on the internet
about UEFI. Here is something important you should understand: 95% of
it was probably garbage."