Re: Do I need to act on this warning from apt when installing grub?

[Pascal Hambourg]

On 16/12/2022 at 15:32, Pascal wrote:
> does this new summary, which takes into account the previous feedback, seem
> "better" to you ?

Yes, better.

> Is the second remark² correct ?

I guess so, as long as the required drivers are embedded in the core 
image and the drive can be read via the BIOS/UEFI. But your diagram of 
EFI boot still shows in inside the ESP.

> is the boot sector of the image directly/readily accessible (eg. editable
> with hexedit) in the boot sector ?

Not sure I understand what you mean. The MBR is accessible just like any 
other sector.

>              INTEL/MBR                GPT
>             PARTITIONING          PARTITIONING
>        ┌─────────────────────┬─────────────────────┐
>        │MBR 1/3              │MBR 1/3              │
>        │POST-MBR-GAP 2/3     │                     │
>        │┌───────────────────┐│┌───────────────────┐│
>        ││PART               │││PART BIOS-BOOT 2/3 ││
>        ││/boot/grub/ 3/3    ││└───────────────────┘│
>   BIOS ││...                ││┌───────────────────┐│
>   BOOT ││                   │││PART               ││
>        ││                   │││/boot/grub/ 3/3    ││
>        ││                   │││...                ││
>        │└───────────────────┘│└───────────────────┘│
>        ├─────────────────────┼─────────────────────┤

Note that this is only the standard layout. Other layouts are possible 
such as:
- a primary boot loader installed in the MBR and chainloads GRUB boot 
image installed in a partition boot sector (PBR)
- core image installed in a Btrfs filesystem reserved boot area
- core image installed as a file /boot/grub/i386-pc/core.img (using 
block lists, unreliable)

> # in BIOS mode :
>    - 1/3 BIOS load and run GRUB boot loader located in the MBR

"GRUB boot image" would be more accurate. "GRUB boot loader" is the 
whole GRUB, not only the boot sector part.

>    - 2/3 GRUB boot loader load and run GRUB core image stored in
> POST-MBR-GAP (INTEL/MBR) or on BIOS-BOOT partition (GTP)¹

GPT.

>    - 3/3 GRUB core image find and mount the GRUB folder², load configuration
> etc..

GRUB does not really mount filesystems. It just reads them.
s/folder/directory/. There is no such thing as "folder" in usual 
filesystems.

> # in UEFI mode :
>    - 1/2 UEFI³ load and run GRUB boot loader located on ESP⁴

"GRUB core image" would be more accurate. "GRUB boot loader" is the 
whole GRUB (including modules in /boot/grub), not only the core image.