How to add second linux distro (xubuntu) to grub?

Question

I installed Xubuntu on a pcie ssd to use side-by-side with Mint. But, I still want to use Mint on its current SATA ssd drive. Ideally, I'd like to make sure that the Xubuntu installation is bootable (I haven't been able to yet), and then have an entry for each in Grub. I have a boot-repair USB for executing any grub commands or generating more info.

I've tried several different combinations of grub installations, but so far no luck. Is there a canoncial way to add a xubuntu distro to grub, assuming I want my current drive (Mint/SATA) to handle grub management?

Here's my lsblk output (excluding snap/loop devices)...

sda           8:0    0 111.8G  0 disk 
└─sda1        8:1    0 108.4G  0 part /
sr0          11:0    1  1024M  0 rom  
nvme0n1     259:0    0   477G  0 disk 
└─nvme0n1p1 259:1    0   477G  0 part /media/alpha/xubuntu

sda1 is where my current Mint distro lives.

Output of update-grub:

$ sudo update-grub
Sourcing file `/etc/default/grub'
Sourcing file `/etc/default/grub.d/50_linuxmint.cfg'
Generating grub configuration file ...
Found linux image: /boot/vmlinuz-4.15.0-66-generic
Found initrd image: /boot/initrd.img-4.15.0-66-generic
Found linux image: /boot/vmlinuz-4.15.0-58-generic
Found initrd image: /boot/initrd.img-4.15.0-58-generic
Found linux image: /boot/vmlinuz-4.15.0-20-generic
Found initrd image: /boot/initrd.img-4.15.0-20-generic
done

Answer 1

Edit: Seems the cause was that one of the the systems booted via legacy BIOS instead of UEFI. This caused Mint being unable to detect the secondary system on nvm.

So for people having a similar problem: be sure all your Linux installations use the same boot method. If fixing this did not help, check the rest of this post.

• boot both of your Linux installations and apply the steps below - one after the other because when installing a new kernel in one of them update-grub will be called and probably overwrite the settings you made in the other.
• edit /etc/default/grub and set GRUB_TIMEOUT_STYLE=menu to make the menu appear, and GRUB_TIMEOUT to maybe 10 (seconds)
• run sudo update-grub this installs all kernels of your currently running Linux into the menu. After that it adds all kernels of all other Linux installations it finds on the disks currently available. Check if it detected all systems available!
• If the above did not help, run sudo grub-install /dev/sda to install grub into the SSD. But most likely this will not change anything as it already was done automatically.

The Linux where update-grub was called last will be at the top and will boot automatically if GRUB_DEFAULT=0. This may change because update-grub is called automatically whenever a new kernel is installed.

If for some reason the auto-detection does not work, edit /etc/grub.d/ files. But you have to update-grub always from the Linux where you modified these files. This might be a problem when update-grub is called automatically after installing new kernels.

If you want the UEFI boot menu to chose which Linux to boot, simply do it this way:

• boot Mint
• sudo grub-install /dev/sda
• boot Xubuntu
• sudo grub-install /dev/nvme0n1

This will cause a different priority in grub, depending which disk the UEFI booted from. To be sure, don't forget update-grub afterwards.

PS. If for some reason autodetect does not work (with the nvme...) Then you have to edit the files in /etc/grub.d

PPS. If it still does not work and you want to try from a live CD. You will need to chroot into the installed system. This is not trivial. And making it install install menu entries for all your Linuxes might be even more difficult.

BTW I am not sure how Linux handles the grub installation point set with grub-install /dev/sda. How does the connection between grub menu and grub boot-sector code work?