1. Running Linux Mint
2. Do side-by-side install of Ubuntu as another version of Linux to boot
3. Both are left working, but Grub version 2.02~beta3-4ubuntu7.1 is stuck booting to Ubuntu and Linux Mint can't be set back as default boot choice. So you have to manually move to the LM partion to boot it rather than Ubuntu.
A. Take some days off from work and learn about the history of GRUB, and try your hand at editing the files manually. OR,
B. As mentioned in a locked 2015 post in this section:
Code: Select all
sudo add-apt-repository ppa:yannubuntu/boot-repair sudo apt-get update sudo apt-get install -y boot-repair && boot-repair
pasetbin output proved by boot-repair for reference if you're interested: http://paste.ubuntu.com/p/B7pD4XZynT/
I had previously tried ppa:danielrichter2007/grub-customizer but it did not solve the problem.