Re: changing from BIOS to GPT

[Gary Dale <garydale@torfree.net>]

On 05/06/15 03:22 PM, Arno Schuring wrote:
> Hi,

> > Date: Fri, 5 Jun 2015 14:45:04 -0400
 From: garydale@torfree.net
    >
    > I have a computer that was set up with an the older style partition
    > table and wanted to convert it to GPT. Since the first partition 
started
    > at 2048, I figured this wouldn't be a problem. Just use gdisk to 
write a
    > new partition table after stealing some space from swap for an 
EFI boot
    > partition. Then reconfigure grub...

    "reconfigure grub" in this case meant uninstalling grub-pc and 
installing grub-efi, right? And
    that EFI boot partition is mounted on /boot/EFI, is formatted as 
FAT32, and has the correct
    partition type (EF01 in gdisk iirc)?

No actually. I never uninstalled grub-pc. The machine seemed to have 
grub-efi-amd64 all along. What I meant was the more mundane update-grub 
sequence. Also, I've been using EF02 as the gpt partition type for the 
efi partition. So far I've never had a problem with that.


    > [..] created new Linux and Swap partitions that I
    > installed Jessie to. These were extended partitions that gdisk 
converted
    > to primary (it displays them as primary but with the original 
numbers).

    There's no such thing as primary/extended/logical partitions with GPT.

I know. I was just explaining that they were not originally primary 
partitions, in case that made any difference.

    > Now I don't even to get a grub rescue prompt. I've tried reinstalling
    > grub in a chroot after booting with system rescue cd but that didn't
    > work. I've reinstalled grub to /dev/sda but again without success.
    > Update-grub sees the partitions but doesn't give me a bootable 
system.
    >
    > BTW: Grub is the grub-efi-amd64 package.
    >
    > At one point I did get it to boot after using F12 to bring up a boot
    > menu and booting from the first HD, but I haven't been able to 
repeat that.

    That probably wouldn't have worked anyway, as EFI doesn't "boot 
from HD". That "boot from
    HD" option probably tried a legacy boot. Instead, EFI relies on a 
list of bootloaders that's stored
    in nvram. You can use efibootmgr to query or modify this list. 
However, that gives you a nice
    chicken-and-egg problem.

I'm not sure why it worked once and, after it did work once, why it 
didn't work after that. efibootmgr doesn't want to tell me anything 
other than it doesn't work in this mode when I boot using system rescue 
cd. I'm also confused as to how grub-efi-amd64 came to be installed. I 
don't recall installing it at any point along the process. I thought 
perhaps it was what the Jessie installer put in place when I originally 
installed Jessie.

    Modifying the efi boot list can only be done through efi system 
calls, and the efi system calls
    are only available if your system is booted in EFI mode to begin with.
    See 
http://blog.uncooperative.org/blog/2014/02/06/the-efi-system-partition/ 
for a description
    of the boot process.

    There is a default efi bootloader, it should be located as 
/boot/efi/efi/boot/bootx64.efi
    assuming your ESP is mounted correctly (see above). Grub is 
probably installed as grubx64.efi,
    you may want to copy the grub efi loader to this default location 
and try again.

I'll give that a try in the morning. However I'm puzzled as to why this 
is happening. I looked at my regular workstation which also uses a gpt 
partitioned disk and it uses grub-pc to boot. However trying to use 
grub-pc on the other machine also failed to boot.

I would have expected an install of grub to leave the machine in a 
bootable state. However purging grub then installing it fresh doesn't 
seem to do that, whether I'm talking about regular or efi.

This leads me to ask "exactly when do I need grub-efi as opposed to 
grub-pc?" Prior to this problem, I'd never even considered that there 
would be a different grub. Since the workstation I'm using to write this 
reply on uses both grub-pc and gpt, what triggers the need for grub-efi?


    If that fails, your next attempt at fixing this would be to find a 
copy of shellx64.efi on the 'net.
    It's a part of Intel's EFI SDK, but easily available as a separate 
download (eek! downloading
    unsigned binaries from random sites). If you put it in the root of 
the ESP (i.e. /boot/efi/shellx64.efi),
    your firmware/bios may offer you a separate option to boot this 
shell instead. Note the use of
    "may", there is no standard for this.

    That shell pretty much behaves as a dos prompt with tab completion, 
you can try to start
    grubx64 from there. That's pretty all the pointers I can give you. 
The process above is how
    I converted one of my machines from legacy to efi boot. Be prepared 
for a lot of reading, trialing,
    and erroring. Best of luck, Arno

Thanks Arno. Not sure what happened to your reply but it took some doing 
to separate your bits from my original post. Hopefully this gets through 
in better shape.  :)