Ok, thanks for clarifying. Your GM's rule makes no sense, though - the purpose of the disbelief saving throw is to reduce or prevent the negative effects of a hostile shadow spell. Using Phantom Steed as an example, unless you order your summon to attack you, there's no need for you to make a saving throw. None of the other consequences of it being summoned by Shadow Conjuration instead of the normal Phantom Steed spell are affected by the disbelief save. In any case, as I said before, your character already knows that the steed is a shadow creation because your character cast the spell in the first place. Having read page 7 of that document, it sounds like your GM is using this to justify your summons never being solid (and therefore functional) for your character, which is a massive nerf to shadow utility that I would argue is against RAW and RAI.