Re: Grub rescue

[Chris Murphy]

On Mar 26, 2013, at 1:46 PM, David WE Roberts <address@hidden> wrote:
> 
> However to me what I am doing is very simple.
> 
> I am booting grub from MBR on disc /dev/sda.
> 
> It is loading the main part of grub from /dev/sdb1 on MBR disc /dev/sdb.

I can't imagine this is workable at all ever.

a.) the jump code on sda's MBR I don't think can jump to another device 
entirely, just to another LBA on that drive.

b.) Installing grub to ext partitions isn't recommended, hence I don't expect 
it's supported. The only way to get it there would be if core.img is already in 
/boot/grub on sdb1, and you use --force with grub-install to place the 
blocklist into the ext VBR.

In any case there isn't a way for grub-install to put the MBR jump code on one 
drive but install core.img somewhere else. The prescribed manner is they both 
go on the same disk, and core.img goes in the MBR gap.

> 
> However I am moving my /dev/sdb from a 750GiB MBR drive to a 3GiB GPT 
> drive.

In that case the GPT drive needs a new partition, 1MB in size is fine, using 
either gdisk code EF02, or parted flag biosboot. Either method creates a BIOS 
Boot partition that grub-install will embed core.img into. That's where this 
code belongs on GPT disks. Not in a file systems's VBR. It doesn't matter where 
it is on the disk, grub-install will find BIOS Boot partitions and use it.


> I am still booting initially (first bits of grub) off MBR disc /dev/sda 
> with the expectation that grub will be able to see /dev/sdb2 on the GPT 
> disc and find /boot/grub with all the information it requires to complete 
> booting.

Expectations are flawed.

> 
> I have assumed that this is what 'part_gpt' is supposed to do. i.e. it 
> should be able to recognise a GPT disc, recognise the partitions within 
> that disc, and then enable grub to access data from one of the partitions.

According to your ls, it's not even seeing the 2nd disk at all let alone its 
partitions. It's only seeing (hd0) and that indicates a BIOS limitation.


Chris Murphy