Re: How to install grub onto an added drive?

[Randy Goldenberg]

On Sunday November 17, 2019, Chris Green wrote: 

[...]

> 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.
>
> 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?

Someone willing to help would need to know if you're using legacy BIOS booting, 
or UEFI.
> Basic questions:-
>
>    Presumably the disk where the /boot filesystem is has to be marked
>    bootable using fdisk.

That depends on how you're booting.  Legacy, or UEFI?
>    How do grub-install and grub-mkconfig relate to each other?  Which
>    do I run first?  Do I need to run both?  What do I need to tell
>    them (parameter-wise)?

grub-install installs grub.  grub-mkconfig writes a grub.cfg file, read by grub 
at boot time.  Parameters are explained in "info grub".
>    Is there anything else I need to do?
>
>    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?

Back up your system.

https://www.clonezilla.org/
> 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.

See "info grub".

Your description of your situation suggests that you are using legacy BIOS 
booting.  If that is the case, in the absence of limiting circumstances, I 
strongly recommend moving to UEFI.

HTH,

Randy