Re: Grub rescue

[David WE Roberts]

On Sun, 24 Mar 2013 15:51:46 -0600, Chris Murphy wrote:

> On Mar 24, 2013, at 1:56 PM, David WE Roberts
> <address@hidden> wrote:
>> Is there any reason why GPT support could not be included as standard?
>> 
>> I am probably swimming in a very restricted pool, but it seems that the
>> main two disc partitioning schemes at the moment are MBR and GPT so if
>> I can update core.img with a GPT handler then it mustn't be too hard to
>> ship this as standard.
> 
> What you're talking about is more practical on UEFI systems where
> core.img appears as an EFI boot services application. Since the
> resulting grubx64.efi is stored as a file on the EFI System partition,
> it's limited to the free space on that partition, which is between
> 100MB-200 typically. Whereas the MBR gap on MBR disks is at best 1MB
> with a modern partitioner. And on GPT, the issue is you need to
> prequalify the specific BIOS with GPT since some BIOS firmware
> implementations get sand in their crack over GPT.
> 
>> Perhaps nobody else is trying to add a big disc to a dual boot
>> configuration and change it from MBR to GPT.
> 
> If the big disk is just a data disk, it should be GPT from the outset.
> Conversion may have some slight benefit as GPT is more well specified
> than MBR, has backup structures, and use CRC to validate the header. But
> as a boot disk, unless otherwise tested, on BIOS hardware MBR should be
> used. On UEFI hardware GPT should be used. Depending on firmware, you
> have a totally different core.img anyway.
> 
> 
> Chris Murphy

Thanks for that.

A whole new world of complications has opened up before me :-)

The hardware in question has EFI capability but is running at the moment 
in legacy mode.

The big disc has been formatted as GPT because it is mainly intended for 
data and that seems the way to go for the reasons you stated.

However the primary MBR SSD has really only space for one operating 
system, which is Windows 7, so I need to be able to boot my luxuries off 
the GPT disc.

However I don't want at the moment to get into UEFI booting off GPT discs 
- just be able to fire up the initial bits of Grub from my MBR disc and 
have the option of booting Windows 7 off MBR or Ububtu off GPT.

Oh, to be clear the current configuration has a 750GB 2.5" disc destined 
for my laptop but pressed into temporary service as the second disc on my 
new(ish) build.

Now the 3TB [when did the notation move from GB to GiB?] permanent data 
disc has turned up I need to move my partitions across from an MBR disc to 
a GPT disc but keep the booting organisation effectively the same.

So I seem to be stuck in a transition between old and new disc formats, 
and old and new BIOS formats.

I could start all over again, have the 3TB disc as my main disc, UEFI 
booting with GPT, and use the SSD as a cache.

However, shan't!

;-)

I think I am in the usual position of the traditional Irish directions:

"If I was going there I wouldn't start from here."

I am starting to suffer temptation to convert the SSD from MBR to GPT, 
change the BIOS settings to UEFI, and go all modern.

Time for bed, I think.

Cheers

David