Im currently running Storm King Thunder and my player are gearing up to attack the cloud giant castle.  The item they are looking for is in a Secret  Chest that appears human sized (a giant cast the spell so it is actually a giant size chest) If they cast dispell magic on the small chest all the items are lost in the ethereal plane.  What happens if they cast the knock spell on the small chest?  Will it open and nothing be in there? 

I'm trying to plan around this and even gave them a hint about this being the greatest obstacle. Thanks in advance

I don’t know the module, so I’m. It sure RAW how it should work, is it magically locked? Or are they supposed to pick a lock? Sorry about not being sure there. 
But even without knowing, I can say, you’re the DM, make it be however you like. The D&D police won’t come knocking on your door. Maybe you give them hints that it should be done a certain way, if they do it that way, they get the reward, if not, they don’t. If they do it partly right, only some of the items remain. 

The human-sized chest that they're casting this on I presume is the smaller "replica" that is called out in Secret Chest. The caster touching the replica is the trigger for summoning the real (giant sized) chest, it is not the chest itself. Casting Knock on the replica would not in any way interact with the hidden Giant chest. Nothing says that the smaller replica can't also be a functioning chest, so Knock on it might very well open the lil' guy... but the stuff inside the Giant chest isn't inside, it's off in the ethereal plane still.

As for Dispel Magic on the replica? Yeah, dangerous stuff, I'd figure that would probably sever the link between replica and Giant chest, leaving it stranded in the ethereal plane and "irretrievably lost" per the spell.

Even if they break the chest, nothing stopping you from plot-magicking another way to retrieve it, or having the chest rematerialize and explode off the edge of the castle or something instead of staying in the ethereal, raining its contents down into a forest far below, etc etc etc... if you really want to give the players a hint that it's a Secret Chest replica, I suggest letting the character that investigates it for traps notice that it looks like a chest but is really just a solid block of wood carved in exquisite detail to look like a chest, right down to having hinges and a lock and everything bolted onto it, but not actually being hollow/functioning. Detect Magic that shows a "conjuration" glow instead of "enchantment" or "illusion" might help too... but yeah, they're probably still just going to try to smash it open with Dispel Magic after they cast Detect Magic and see it glow, players are usually pretty blunt. 

 Hold up. Secret chest has a duration of instantaneous. That means it's effect cant be detected after being cast or ended early, but the spell description includes several ways the spell can end early and what happens if it does. So it should have a duration of until ended or dispelled. Damn rules, doesn't anyone proofread these?

I've hinted that the chest will be a problem.  I like the idea of saying it is carved to insane detail but isn't an actual functioning chest. They will have to convince or force the caster to summon it. Even if they kill the caster, they would be able to resurrect her and have her. 

They have been given a hint that they will have a problem with it.  I left it vague on purpose. There are several ways for then to achieve their goal. I was mainly looking what happened to the replica if certain spells were used. 

Quote from DxJxC >>

 Hold up. Secret chest has a duration of instantaneous. That means it's effect cant be detected after being cast or ended early, but the spell description includes several ways the spell can end early and what happens if it does. So it should have a duration of until ended or dispelled. Damn rules, doesn't anyone proofread these?

 I’m not sure why this is an issue.  The ending conditions for the effect are plainly spelled out and fit with the “specific beats general” rule of D&D.  Besides the rules for instantaneous duration spells say nothing of the effect not being able to be detected or ended early, just that the effect can’t be dispelled (presumably by dispel magic ).  None of the conditions for ending the effect involve dispelling the effect.

Instantaneous

Many spells are instantaneous. The spell harms, heals, creates, or alters a creature or an object in a way that can't be dispelled, because its magic exists only for an instant.

It seems to me you're overthinking things. Casting Dispel Magic on the replica chest doesn't do anything. The Secret Chest spell is cast upon the actual chest, not the replica; the replica is just the material component, not the target. Casting Dispel Magic on the replica chest does not destroy the object, so that doesn't trigger the spell ending on the actual chest either.

So it's your position that there is nothing magical about the replica while it is linked to an extra dimensional chest? Agree to disagree on that I guess. 

Quote from Chicken_Champ >>

So it's your position that there is nothing magical about the replica while it is linked to an extra dimensional chest? Agree to disagree on that I guess. 

 Not in the sense of how it is germane to the mechanical interaction with these spells. Consider this: If you were to cast Dispel Magic on an Arcane Focus I'm holding, would that lead to any spell for which that item served as the material component to fizzle?

The replica in Secret Chest is part of the spells ongoing magical functions, not just a component that was used in the initial casting. No, I wouldn't normally undo spells that were previously cast with the focus just because it's later dispelled, but yes, I would undo any spells that were still actively linked to interaction with the focus (can't think of an example of such a spell, which is a good indication of why a Focus isn't a good analogue for a Secret Chest's replica). 

Secret Chest, be it the chest or replica, cannot be dispelled. So, isn't this arguing about dispelling it kinda pointless?

Source? 

Oh, the Instantaneous rules in Chapter 10?

Instantaneous

Many spells are instantaneous. The spell harms, heals, creates, or alters a creature or an object in a way that can't be dispelled, because its magic exists only for an instant.

Yeah, like Dx said, that's a headscratcher, because the spell very clearly isn't instantaneous, as (1) the spell description of Secret Chest discusses what happens when "the spell ends," and (2) nothing about the spell "harms, heals, creates, or alters a creature or an object" in a way that is appropriate to label as Instantaneous, and (3) logically, we're pretty clearly talking about a replica that is carrying an active magical link with a hidden chest, ludicrous to presume that that isn't a "spell on the target" replica that Dispel Magic couldn't sink its teeth into, or Detect Magic pick up on.

Like Dx said... spell doesn't make sense. Even reading it strict RAW, I think there's room to argue that the spell's description calling itself out as having an ongoing spell in danger of ending operates as a specific exception to the Chapter 10 rules about Instantaneous-duration spells in general. 

Quote from Chicken_Champ >>

Source? 

 
Duration: Instantaneous.

Iconarising has quoted the rules (from Chapter 10) above so I won't do it again. Instantaneous spells cannot be dispelled unless the specific spell says otherwise. 

Sorry edited my post, see above

Regardless of whether you agree the duration is instaneous or not is irrelevant by RAW. The spell is listed as Duration: Instantaneous. The RAW reason is that the effect of the spell is now an inherent nature of the item and not because of an ongoing spell effect. There's a difference between spell effect (only during an active spell) and a magical effect (anything magical). 

The spell's lasting effect is a "magical effect" not a spell effect. The chest is no longer "under the spell", and dispel magic only removes "spell effects".

Quote from Chicken_Champ >>

The replica in Secret Chest is part of the spells ongoing magical functions, not just a component that was used in the initial casting. No, I wouldn't normally undo spells that were previously cast with the focus just because it's later dispelled, but yes, I would undo any spells that were still actively linked to interaction with the focus (can't think of an example of such a spell, which is a good indication of why a Focus isn't a good analogue for a Secret Chest's replica). 

I'd say that's a good indication of why the replica chest isn't any different from the focus. I'm not saying there isn't a knee-jerk inclination to think dispelling the replica would do something, but the RAW on both of the spells do not function that way. You can think of it in the same way that Dispel Magic doesn't affect anything with an instantaneous duration (which addresses DxJxC's concern too). Secret Chest is instantaneous, not an ongoing effect. It is the same as Find Familiar; once the spell is cast, there is no active magic left to dispel. It's an object/creature with a magical quality, not an object/creature sustained by magic (like Conjure Woodland Beings).

Could the description of Secret Chest be better? Yeah, absolutely. Can it be described in such a way that it looks like an edge case? Yeah, that's the specialty of this forum. Does it matter? No, the spell's duration is instantaneous, and there's no ambiguity about what that means when it comes to Dispel Magic.

I agree, basically this spell creates a magical item; one that has specific (and tenuous) triggers for the effect to end.

I'm not disagreeing about the meaning of Chapter 10. But specific beats general, and Secret Chest specifically calls itself out as not working as other Instantaneous spells do. Order of operation:

1. Chapter 10 says I cannot "dispel" (lower case "d", doesn't say I can't cast Dispel Magic, just not dispel in general) a spell that is instantaneous, "because its magic exists only for an instant."
2. Secret Chest is cast, which has a duration of instant. The spell's description describes creating an ongoing "spell's effect", that that effect will end if the "spell ends," that the caster is still linked to the spell actively because they can "end the spell as an action," and that the replica is still linked to the spell actively because the effect ends if "the smaller replica chest is destroyed." These four things, whether read separately or all together, operate as a specific exception to the general rule that the magic of an instant spell "exists only for an instant."
3. Chapter 10 provides no general rule that magical effects that linger cannot be dispelled. The instant section only said that instant spells can't be dispelled because there isn't a lingering effect; where that is demonstrably untrue due to a specific exception, it does not claim to control.
4. Dispel Magic does not require a non-instant spell. It can target a "magical effect." Secret Chest specifically describes itself as having an ongoing "spell's effect," and it is fairly logical that this effect can be found on the replica considering that destruction of the replica ends said effect. Dispel Magic also describes ending a spell "on" a target object. When a caster casts Secret Chest, the spell is cast by touching both the big chest and the replica, and it is fairly logical that the spell has been cast "on" both objects (again, especially in light of the replica's continued essential connection to the ongoing spell effect). Whether you're targeting an ongoing "magical effect" or an ongoing spell that is "on" the replica, both seem to exist. 

The specific language of Secret Chest serves as an exception to Chapter 10, by providing that this spell does have an ongoing magical effect which can be ended. The specific language of Dispel Magic serves as an exception to Chapter 10, by providing that is able to target "magical effects", which Secret Chest specifically says that it has. Ergo, both of these spells have a specific interaction with each other that is more specific than Chapter 10's general prohibition on dispelling instant non-lingering effects. Whether you argue that I'm dispelling Secret Chest or the separate ongoing magical effect left behind after a casting of Secret Chest comes down to semantics, but either way, the replica clearly can be effected by a casting of Dispel Magic. 

To be a little clearer, there are two alternate overlapping reasons you can target the replica: you can cast Dispel Magic to end a spell, or you can cast Dispel Magic to end a magical effect. Chapter 10 only contains rules that prevent dispeling spells, and doesn't talk about or claim to limit the ability of a lasting magical effect to be dispelled. 

1. Secret Chest the spell hasn't really ended despite being Instantaneous and non-concentration, because its own description says it hasn't, and so Secret Chest the spell can be dispelled as a spell "on" an object.
2. Regardless of whether the spell has ended or can be displled, the spell has created and left behind a lasting "magical effect." Dispel Magic can specifically target and dispel magical effects, regardless of whether those effects were created by a spell or whether that spell itself could have been dispeled while active.  (see edit below, guess all my eggs are in #1's basket)

Edit: Actually, I'm reading it closer... and though Dispel Magic can target magical effects regardless of whether they involve an active spell, all that a casting of the spell explicitly does mechanically is "end a spell," not an effect itself. In practice this is 100% not the way that this spell works in play, I'm sure there are countless examples of its use in adventures describing otherwise, but I did not quite realize that RAW Dispel Magic does nothing against non-active-spell magical effects. Huh, I guess to do that you have to cast Antimagic Field, dumb.