Getting mods working for games
Yeah mods can be quite troublesome.
If you use a hard drive other than your os install drive then you need to go to the steam website to get the installer and not use the one in the built in app store.
Sounds like a steam problem.
Non gaming related I've had numerous issues trying to manage permissions for my hard drives
Eh, i remember mounting being a bit troublesome a few years back, but current GNOME should take of that for you with very little input on your end. This brings us to PopOS 22 which is starting to get really old at this point, I'd consider moving away to something that's not left abandoned while they finish up Cosmic.
There's always plenty of good reasons to postpone something and only one to do it now.