If you installed PCManFM on Void Linux or another minimal setup, here are the steps you need to follow to get USB-automounting to work properly:

  1. Install the optional dependency gvfs, which should come with udisks2 and a polkit
  2. Launch your WM session with exec dbus-run-session WMHERE so all these programs can talk to each other.
  3. Make sure the policykit service is running.
  4. Add the following file to /etc/polkit-1/rules.d/ as 50-udisks.rules or something:
    polkit.addRule(function(action, subject) {
       if (subject.isInGroup("wheel")) {
            if (action.id.startsWith("org.freedesktop.udisks2.")) {
                return polkit.Result.YES;
            }
        }
    });

    Credit to fractalf on GitHub for these concise rules.