Re: How does one add a second Ubuntu installation to an existing one?

[Pascal Hambourg]

Le 21/05/2021 à 10:24, Chris Green a écrit :
> How should I do the changes to /boot/grub/grub.cfg?  Do I edit
> /etc/default/grub and then run grub-mkconfig?  However I don't really
> see how this would work, I can set the output file for grub-mkconfig
> but how do I tell it to use the /etc/default/grub in /dev/sdc1?

You can either run grub-mkconfig or edit /boot/grub/grub.cfg manually.
If you choose to use grub-mkconfig, you must run in while /dev/sdc1 is 
the root, either by booting it or by chrooting into it from the original 
system.

> At what stage does the copy in /dev/sdc1 get added to my boot menu?

After you run update-grub or grub-mkconfig in the original OS.

> > I would also install GRUB on /dev/sdc from the copy to be able to boot if
> > the original installation is so broken that even GRUB does not work any
> > more.
> Definitely, is that just a matter of booting the copy OS and running
> grub-install?

You don't even need to boot (or chroot) into the copy OS. You can run

# grub-install --boot-directory=/mnt /dev/sdc

in the original OS.