Re: How to install grub onto an added drive?

[Pascal Hambourg]

Le 17/11/2019 à 20:20, Chris Green a écrit :
> I want to add a new SSD to my current desktop system.  This in itself
> isn't a major problem, I've done similar things before without
> problems (in fact I did this already with the current small and
> relatively slow SATA SSD that has the / filesystem on it).  The issue
> is that I suspect the motherboard won't be able to boot from the new
> NVME/PCIe SSD so I'm aiming to have a small, bootable drive to just
> provide the boot files and have everything except for /boot on the
> new, fast, SSD.

If the motherboard has an M.2 slot with NVMe support, the firmware 
should be able to boot from an NMVe SSD. If it has no M.2 slot and you 
added the NMVe SSD with a PCIe adapter card, it is unlikely the firmware 
is able to boot from an NVMe SSD.

> So, I can move all the required OS files to the new hard disk but how
> do I get grub installed on whatever I have as a 'small' boot disk?

You must install not only GRUB but the whole /boot on the boot drive.

> Basic questions:-
>
>      Presumably the disk where the /boot filesystem is has to be marked
>      bootable using fdisk.

You cannot mark a whole drive as bootable. You can mark a primary 
partition as bootable, but this is needed only with broken firmwares 
which require it. A compliant BIOS does not require a bootable partition.

>      How do grub-install and grub-mkconfig relate to each other?

grub-install installs GRUB in the specified locations and grub-mkconfig 
generates a configuration script on its standard output that you can 
write to the location where GRUB expects to find it, by default 
/boot/grub/grub.cfg. update-grub must be run from the target system 
(natively or chrooted). grub-install can be run from any other system if 
you specify the --boot-directory= location.

>      Which do I run first?

It does not matter.

>  Do I need to run both?

Yes.

>      What do I need to tell them (parameter-wise)?

In BIOS/legacy mode, grub-install takes at least one parameter : the 
device node you want to install GRUB on. grub-mkconfig takes no parameter.

>      Is there anything else I need to do?

Update fstab and the initramfs.

>      I guess I need to run grub-install and grub-mkconfig on the system
>      as I want it configured, i.e. with the new/small disk waiting for
>      grub to be installed on /boot.  So, this feels a bit risky as,
>      until grub has been installed there the system won't [re]boot.
>      What's the best way to make sure I have a 'get out' if it all goes
>      pear shaped?
>
>
> Yes, I know that question about grub-install and grub-mkconfig seems
> rather naive but I have to say none of the tutorials, man pages or
> other help that I could find actually clarified this.