Re: Grub rescue

[David]

On Tue, 26 Mar 2013 14:48:35 -0600, Chris Murphy wrote:

> 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.
> 
<snip>

Firstly ""It Just Works[TM]"

Secondly, your rebuttal seems over complicated.

The MBR on /dev/sda holds core.img.

No force options were used in this picture and no animals were harmed.

As far as I know the initial invocation of grub from the MBR is wise 
enough to find /boot/grub on /dev/sdb1 and then get all the required 
information from there to put up the boot menu.

Having just (due to finger trouble) set the prefix to /grub instead of /
boot/grub and dropped back into the shell prompt I can say that setting 
the prefix correctly and invoking normal does get to the boot menu.

So you seem to be telling me that I can't do what I obviously can.

If I put another HDD into my PC with a version of Ubuntu on it, then 
update-grub will find this, include it in the boot menu, and it will boot.

Been there with a PATA disc.

So I am somewhat bemused by you saying that it can't be done/ isn't 
supported.

If you have several MBR discs in a PC you should be able to install Linux 
(Ubuntu) onto any of those discs and still boot up from grub on your 
chosen boot drive.

The Ubuntu install is quite happy to have a single partition for all the 
data - /boot/grub is by definition found under / - and just asks which 
disc should hold the initial boot code.

So with boring old MBR discs everything works.

My problem is that grub doesn't seem to be able to see GPT discs.

See my long post for the Boot Info.

I don't know if the fact I am on Grub2 (v1.99) has any bearing on all this.

Cheers

Dave R