I currently run Mint Mate 20.1 and gave the EndeavourOS 2021.08.27 ISO a try and…
it mucked up my Mint Mate
now my external drives no longer auto-mount and my Disk Mounter Applet in the panel now shows root and efi that should not be there
other than having to manually mount all my drives Mint Mate appears to be working as it should…
A little tough to tell what happened without information on the current disk setup. Did you install using manual mode, or did you choose an automated install?
Stuff we might need includes the output of lsblk and perhaps inxi -Faz. Copy the output from the terminal CtrlShiftC - add 3 `, then paste into a message, and follow the paste with another 3 1 (for each of the items). Once we see that we might be able to guess what’s going on, and possibly clean it up…
no one knows… 
but as usual I figured it out myself 
$ sudo update-grub
now things are back to normal
the Live EndeavourOS ISO mucked with my current grub - WOW! 
I’m just going to have to wait until I get another computer to mess with Arch based distros, EndeavourOS joins Manjaro in failure with my Dell system
and I was so wanting something Arch
Welcome to the forum 
.
Grub no longer checks for other installed OS by default. Install os-probe
sudo pacman -S os-probe
You then need to edit /etc/default/grub to add or uncomment GRUB_DISABLE_OS_PROBER=false
Then re-make the grub config.
sudo grub-mkconfig -o /boot/grub/grub.cfg
Also,
Arch based systems have two .img files in the “initrd” line of the grub config.
For example, when using an intel microcode image, this kind of initrd line should be generated by grub-mkconfig:
initrd /boot/intel-ucode.img /boot/initramfs-linux.img
but instead, this is generated by debian based version of grub:
initrd /boot/intel-ucode.img
The easiest solution is to use EndeavourOS grub but if you want to continue using your existing version of grub, you will need to edit the “initrd” line of all “menuentry” s for Arch based systems.
Alternatively, you could disable or uninstall os-prober and manually create the entries for your Arch based systems in /etc/default/grub.d/40_custom
@kjw, the OP is not using EndeavourOS. He is using Linux Mint. pacman does not work on Linux Mint 
how can using a live iso mess up the installed grub?
I assumed the op meant they had given it a try by installing it alongside their existing OS.
I don’t think it can. 
The ISO is not using grub at all, it boots on systemd-boot if EFI and syslinux if Bios-legacy…
As long as user do not start anything or run any command the LiveISO is not doing anything to the system.
But it is possible that user muck up his system. as you can do almost everything from the live session… like deleting partitions or removing folders, overwriting.
–closing–