If you are unable to mount other disk partitions from nautilus running in a custom environment with different Window Managers like Xmonad or Awesome, you might have to install appropriate polkit agent available with package manager and autostart the polkit agent by adding it to startup scripts the path would probably look something like this /usr/lib/polkit-gnome/polkit-gnome-authentication-agent-1.
I use nautilus as file manager in a custom Xmonad environment which is dependent on gnome stuff so I installed polkit-gnome, for more information here's a link to arch wiki Polkit Article
Link to my Xmonad configuration for interested folks, I have remixed lot of configs that I have found online