Thanks for the answer. However, things aren't that easy :
and btw, the code I've posted in my first post leads to
ubuntu@ubuntu:~$ sudo grub-install /dev/sda
/usr/sbin/grub-probe: error: cannot find a device for /boot/grub (is /dev mounted?).
[UUID]=UUID of the partition containing linux. The same message appears if the root-directory option points to the efi partition...
ubuntu@ubuntu:~$ sudo grub-install --root-directory=/media/[UUID] /dev/sda
/usr/sbin/grub-setup: warn: This GPT partition label has no BIOS Boot Partition; embedding won't be possible!.
/usr/sbin/grub-setup: warn: Embedding is not possible. GRUB can only be installed in this setup by using blocklists. However, blocklists are UNRELIABLE and their use is discouraged..
/usr/sbin/grub-setup: error: if you really want blocklists, use --force.