Le 08/10/2025 à 23:30, Antoine a écrit :
> On Wednesday, 8 October at 15:44, Didier Kryn wrote:
>>
>>     Is the bigger part of Grub stored into a non-formatted partition,
>> or a free space of the device (some space which is not allocated by
>> the partition table)?
>>
>>     As I could read on the Internet, if you have a DOS-type, also
>> called MBR, partition table, you need to preserve free space before
>> the first partition,
>
> Indeed. On an MBR-partitioned disk with "grub legacy", you need a bit
> of reserved space (the "MBR gap") before your first partition. This is
> where you have a "stage 1_5" image which allows grub to read a
> filesystem (there are therefore several such images, one for each type
> of filesystem).
>
>> which is what I've done since the advent of Grub2, but if you use a
>> GNU Partition Table, you need a Grub-dedicated non-formatted partition.
>
> This is also true (except that it's called a GUID Partition Table, I
> believe).
>
> There's a bit of a catch, though.
> It's possible to have a GPT-partitioned disk on a BIOS computer or on
> a UEFI computer.
>
> On a UEFI computer, the EFI firmware boots into the partition flagged
> "boot" (or maybe "esp"? I'm not entirely sure about this).
> The GPT format also works on BIOS computers since it has a
> "protective" BIOS table which makes it look enough like an
> MBR-formatted disk that the BIOS is fooled, at least long enough for
> bootup. This means a GPT-disk can have grub 2 installed on it as if it
> were an MBR-disk and boot from BIOS. In this case, grub 2 doesn't use
> the MBR gap. Instead it uses that unformatted partition (to prevent
> overwriting).
>
> I have less experience with UEFI computers, so I invite anyone who
> knows more to pitch in.
>
>> However I don't know how the last is possible, because my little
>> experience with GPT is that you need parted to crate it and parted
>> automatically formats all the partitions.
>
> I just checked my copy of parted and I was able to create an
> unformatted partition, simply by not giving it a filesystem type.
>
>> Is there a specific Grub partition type?
>
> There is, on GPT disks. It's defined by the "bios_grub" flag.
>
>>     Does anyone have a comprehensive explanation?
>
> That's aboout all I know on the subject. Does anyone else know more ?
>
> - Antoine
>
    You are right for GUID of couse (~: It was stupid from me to
imagine Intel/Windows would tolerate anything GNU.

--    Didier