I recently upgraded to the 1804-based KDE Neon. However, before doing the upgrade, I cloned my existing 1604-based Neon root partition (using dd) to another partition on my internal drive, just in case there was a problem with the upgrade. Actually, I booted into the cloned partition and ran the upgrade on that, leaving the original 1604 installation untouched. Before running the upgrade, I had no problem booting into whichever of the two (the original or the clone) from the GRUB menu (having run update-grub after cloning).

After running the upgrade to 1804, my computer always boots into the old 1604 installation, regardless of which item I choose from the GRUB menu. Both the default GRUB entry, and the "KDE Neon 1804" entry boot into the same OS. The only way I can boot into the upgraded OS, is to set the root partition, kernel and initrd manually in the GRUB command shell. If I do that, I can boot into the upgraded OS on the cloned partition and everything works normally. The only problem is that I have not found a way to boot into the upgraded OS from the regular GRUB GUI. Even after I ran update-grub while booted into the upgraded OS, the computer still reboots into the old OS by default.

Admittedly, it is sometimes hard to tell which OS I am booted into (because they look the same). Running lsblk allows me to identify which partition is mounted as root. Another indication is the presence or absence of distro upgrade notifications in the panel.

None of the usual commands, such as

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update-grub

or

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grub-mkconfig

, have fixed the problem. Do I need to manually edit the grub config?

My system is EFI boot. There are two more OSes on it besides the two NEON versions mentioned above.