How to install grub onto an added drive?

[Chris Green]

This is maybe going to be a rather confused and long question but I
will try and make it as clear as possible.

I am a long time Linux user and have been using Unix[like] systems for
40 years or so.  I currently run xubuntu on several systems.

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?

Basic questions:-

    Presumably the disk where the /boot filesystem is has to be marked
    bootable using fdisk.  

    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)?

    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?


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.


-- 
Chris Green