Re: Grub rescue

[David WE Roberts]

On Wed, 27 Mar 2013 13:25:05 -0600, Chris Murphy wrote:

> On Mar 27, 2013, at 3:21 AM, David WE Roberts
> <address@hidden> wrote:
> 
>> As posted elsewhere, with all 3 discs connected and grub-install run
>> against a GPT partition, on reboot I can see the two MBR discs and not
>> the GPT disc.
> 
> This is all very unclear terminology, as I mentioned elsewhere.
> 
> This GPT disk contains a BIOS Boot partition? And you used grub-install
> on that GPT disk? And when you reboot you're absolutely certain it's
> loading GRUB off the GPT disk?
> 
> It doesn't make sense that GRUB loads from a GPT disk, and then can't
> see that GPT disk. That's pretty inexplicable, don't you think?
> 
> 
>> Because core.img has been built with part_gpt I can't see the
>> partitions on either MBR disc.
> 
> It sounds like os-prober isn't finding systems on those MBR partitioned
> disks, therefore it's not building MBR support into the core.img. This
> might have to do with an old version of os-prober, or maybe the
> partitions os-prober is searching are encrypted.
> 
> 
>> I note that the version (1.99) of grub2 is quite old - this is the
>> version that ships with the latest version (12.04) of Ubuntu.
> 
> 12.10 has 2.00-7 listed in the repos. I suspect you can still use it on
> 12.04. I don't know if os-prober is a separate package, but I'd make
> sure that too is as up to date as you can make it.


We seem doomed to exchange messages but completely fail to communicate.

Throughout all of this grub *always* boots from the MBR on /dev/sda.

The problem is that under certain circumstances it can't find /boot/grub.

As Jordan has said, if when you run 'grub-install' the location of /boot/
grub is on a GPT disc then "part_gpt" will be included in core.img.

If the location of /boot/grub is on an MBR disc then 'part_msdos' will be 
included in core.img.

The initial job of 'core.img' is to get itself up and running and then 
locate the partition where /boot/grub resides and it can then proceed to 
load all the additional modules and set all the variables it needs to get 
ready to boot a variety of OS, and then present the user with a menu.

The copy of 'core.img' stored in the MBR is a pared down version because 
of the lack of available space and the restricted environment it starts in.

Once it has found /boot/grub it can then flesh itself out with all the 
other modules it needs such as for example NTFS support.

Hopefully this makes things a little clearer :-)

Cheers

David