Simple RAW for 5e V: The Disadvantage Strikes Back

[Hairfish]

Q 378a: Can a spell be used with a Shield Guardian's Spell Storing ability if the spell cannot be cast in a way that targets the Shield Guardian? Strictly speaking, the spell doesn't have to 'target' the Guardian, only be cast 'on' it. However, the phrasing that the caster "must" cast the spell on the guardian implies that if a spell cannot be cast on the Guardian, it is an ineligible effect for the ability.

Q 378b: Can a spell be used with a Shield Guardian's Spell Storing ability if the spell would not work on the Guardian? Could you store a Cure Wounds or Charm Person in it?

Outside RAW, I like to imagine you fulfil 'casting a spell on the guardian' by standing on the guardian while casting the spell.

A378: Since the wording avoids the use of "target", it's up to DM interpretation. My take is that any 4th-level or lower spell could be stored this way. Just note that "the spell has no effect" on the initial cast if used in this manner, not "the spell has no effect on the guardian". So the amulet wearer can't Fireball the party fighting the guardian and include the guardian in the blast zone to get a second Fireball.

There's nothing specifying the spell to be stored must be able to affect the guardian, nor that the guardian has to use the stored spell to target itself (unless the amulet wearer set up a contingency to do so). Though if you're going with the strict interpretation that the initial cast must target the guardian, then Cure Wounds can target the guardian (it just can't affect it normally, either) but Charm Person cannot (the target must be a humanoid).