The last stable version of pcmanfm (in portage) needs gnome-base/gvfs with gdu and udev use flags active if you want to mount removable media with pcmanfm. But there isn't a gvfs[gdu] and gvfs[udev] dependence in pcmanfm ebuild. I think would be a good idea to modify the ebuild. Reproducible: Always
Even if, x11-libs/libfm would still be a better place for such changes.
This is fixed in tree...