Re: Is it possible to have grub2's boot.img as my MBR, but have it look in a separate partition for core.img?

[Wesley Smith]

Colin Watson <cjwatson <at> ubuntu.com> writes:

> ...
> Yes, you can do this by using 'grub-install /dev/sda1' (or whatever the
> device name for your /boot partition is).

Thanks for your help, but no, it appears you cannot do this by simply using
"grub-install /dev/sdx". I tried this, and yes, it did make the target
partition bootable (it contained a Grub loader in its PBR). However, to ensure
the disk MBR had also been updated and was now getting its core.img file from
the /boot partition and not from the sectors immediately following it, I built
a dummy track containing just the MBR post-"grub-install" and nothing but zeros
after, and then I copied it to /dev/sda:

dd if=/dev/zero of=/track.image bs=512 count=63
dd if=/dev/sda of=/track.image bs=512 count=1 conv=notrunc
dd if=/track.image of=/dev/sda bs=512 count=63

If the MBR no longer needs the embedded core.img, then it being replaced with
empty sectors shouldn't bother Grub the slightest. Unfortunately, upon reboot I
was greeted only with the word "GRUB" and a blinking cursor, which I took to
mean it couldn't find core.img and was unable to proceed.

It would be nice if Grub2 could both make a partition bootable and also install
an MBR that relied on that partition (rather than the embedded core.img). I
know this is "dangerous," but it is necessary sometimes, and it was not at all
difficult to accomplish with legacy Grub.

> Note that this setup typically requires the use of blocklists, which are
> inherently unreliable.  For example, you may find that you need to
> re-run grub-install after certain filesystem operations; see the "And
> blocklists?" section of http://grub.enbug.org/BIOS_Boot_Partition for a
> description of the problem.  This problem was present in GRUB Legacy as
> well, although it did not warn about it.
>