@ Marcet
Si cela peut aider, en consultant "info grub2" en console, il y a ceci concernant GPT :
GPT
===
Some newer systems use the GUID Partition Table (GPT) format. This was
specified as part of the Extensible Firmware Interface (EFI), but it can
also be used on BIOS platforms if system software supports it; for
example, GRUB and GNU/Linux can be used in this configuration. With
this format, it is possible to reserve a whole partition for GRUB,
called the BIOS Boot Partition. GRUB can then be embedded into that
partition without the risk of being overwritten by other software and
without being contained in a filesystem which might move its blocks
around.
When creating a BIOS Boot Partition on a GPT system, you should make
sure that it is at least 31 KiB in size. (GPT-formatted disks are not
usually particularly small, so we recommend that you make it larger
than the bare minimum, such as 1 MiB, to allow plenty of room for
growth.) You must also make sure that it has the proper partition
type. Using GNU Parted, you can set this using a command such as the
following:
# parted /dev/DISK set PARTITION-NUMBER bios_grub on
If you are using gdisk, set the partition type to `0xEF02'. With
partitioning programs that require setting the GUID directly, it should
be `21686148-6449-6e6f-744e656564454649'.
*Caution:* Be very careful which partition you select! When GRUB
finds a BIOS Boot Partition during installation, it will automatically
overwrite part of it. Make sure that the partition does not contain any
other data.