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I'm using Artix-Xfce-Runit as primary System and had never such Problems with parallel installed LinuxMint, Zorin and now LMDE 6. - No matter if I use it on internal or am external (USB) SSD.
But i'm booting in BIOS-Mode, because UEFI makes me sick.
Hello,
Once Artix + GRUB is installed on GPT, you can install another distribution alongside it.
But when installing this new system, you will not install GRUB (disable the option in the graphical installer).
Once this installation is complete, you restart on Artix and launch :
sudo update-grub
I have often done it