This is my setup for installing:
However, the installer complains:
What is the problem here?
I’m using legacy boot, not UEFI, don’t know if that’s the problem…
You can tell the installer to ignore the problem. If I recall, you just click “Continue”. And the chances are that you will have a good install.
The installer default is to use a GPT partition table, instead of the traditional DOS partition table.
With a traditional DOS partition table, when you install grub to the MBR, it will also install boot code in the gap between the MBR and the first usable partition. But GPT partitioning does not allow that. So the bios_boot partition is a way of giving grub a little extra space for installing boot code.
Since you are using “ext4”, grub can find ways of installing to the MBR that do not depend on that gap or on a bios_boot partition. That’s why it will work anyway. If you were using “btrfs”, then it is a tad more tricky. However, I think that would also have worked, because of a grub update a few days ago.
I’m using legacy boot, not UEFI, don’t know if that’s the problem…
UEFI booting does not need a bios_boot partition. But it does need an EFI partition.
That’s wrong. In grub2 parlance “install on MBR” means core.img is written either in post-MBR gap (on MBR) or in bios_boot partition (on GPT). If this is not possible, installation fails. The only way to leave core.img on ext4 filesystem is to “install on partition”.