Boot confusion

[Liam Proven]

On Mon, 7 Jun 2021 at 20:32, Jack McGee <jack at greendesk.net> wrote:
>
>
> The 250 gig SSD is
>
>
> jack at Desktop:~$ sudo blkid -p /dev/sda
> [sudo] password for jack:
> /dev/sda: PTUUID="562dd49d-ae00-454e-bf2d-b86cd24573e2" PTTYPE="gpt"
> jack at Desktop:~$

OK. That tells us that /dev/sda is GUID
562dd49d-ae00-454e-bf2d-b86cd24573e2 and that it is partitioned with
the GUID partition table, but it does not tell us for sure that it is
the SSD.

The command:

sudo sfdisk -l

... should list all partitions on all drives, and the command

sudo mount

... should tell us what is mounted where.

If the boot drive is GPT, then I think you need a very small partition
of type "GRUB BIOS boot":

https://en.wikipedia.org/wiki/BIOS_boot_partition

...  but I have never tried booting a BIOS machine from a GPT drive so
I am just trying to remember what I've read. This does make it a
little bit more complicated and if anyone else on the list would like
to confirm that for me, I would appreciate it.

As I understand it, this is how it works.

With the traditional legacy boot process, and traditional
DOS-partitioned MBR disks, there is enough space in the Master Boot
Record for GRUB to fit. The rest of GRUB goes in the boot partition
(if you have a separate one) or the /boot directory in the / (root)
partition.

But with disks partitioned with GPT, like yours, there isn't room for
GRUB in the boot record. So GRUB needs a separate, unformatted, 1 MB
(yes, just one meg) partition, with the "bios_boot" flag set. GParted
from a bootable USB key can do this. You could take 1MB off the end of
your root partition, make a new partition and use the GParted "manage
flags" command to set (enable, tick) the "bios_grub" flag.

https://help.ubuntu.com/community/Grub2/Installing

When you do a BIOS-type, legacy, boot, when you install GRUB, it will
recognise a GPT disk and automatically put part of itself in this tiny
1MB bios-grub partition.

That, AIUI, is it.

> does grub need to go in it's own partition?

On MBR disks, no. On GPT disks, yes.

But do make sure that you know how to change the boot device in your
firmware settings first!

A few years ago I used a notebook with 2 hard disk bays. It shipped
with bay 2 empty and a plastic cover over the empty bay. The cover had
internal supporting legs meaning that you couldn't fit a disk in the
bay with that cover, and if you fitted one, you needed a cradle (a
tray) to hold the disk and a different cover.

I didn't have these and it was already an old machine when it was given to me.

So when I tried to fit a disk, I had to saw the supporting legs off.
There was no way to screw the disk into place, as I didn't have a
mounting tray.

I installed an SSD in the 1st bay and a 1TB HDD in the 2nd bay.

Problem! The HDD was not held securely, and if I pressed on the cover
it made a terrifying drilling/grinding noise. I did not want to
destroy my new 1TB disk, so I swapped the SSD and HDD around. HDD in
bay 0, so device /dev/sda, and SDD in bay 1, so device /dev/sdb. The
SSD was very light and had no moving parts, so it was held in place by
just the SATA and power connectors, and pressing on the cover didn't
make any noises at all.

But I wanted to boot from the SDD.

So, I put / on sdb (/dev/sdb1) and /home on sda (/dev/sda1). I
installed GRUB to /dev/sdb.

In the BIOS, I set the bootable hard disk to the 2nd drive (disk 1).

GRUB loaded from /dev/sdb and loaded the OS from /dev/sdb1 and then
mounted /home from /dev/sda1.

Why I am telling you this is that you will need to change your
firmware to check your SSD first instead of the HDD, which from what
you've told us, is what it is currently booting from.

-- 
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