This is different. They are trying to break into a human being, as a way of breaking into a secured hard drive. They are also saying that they know the mind of the defendant.
Not quite. They ARE trying to break into a physical thing, an encrypted device. However, the key is no longer a physical thing. If it's a locked door, they don't force you to turn over the key, they simply break down the door. The key need not be involved.
In the case of encryption, they need the key. So they turn to you.
This becomes a question of "they are authorized to search it, and to use force if necessary, but in this case force isn't effective" They are then taking a step back to the purpose of the law, which is to allow them access to search for evidence. No one seems to be contesting this point, they are allowed, but they are also physically powerless. The route they need to take from there is not the route of force, but the route of key. And that leads them to YOU.
So... it's now a question of "are we looking at the intent of the law, or the (outdated) description?" The kneejerk response is to say "we should follow the letter of the law, not make exceptions based on the intent", because that option appears to offer greater protection to the citizen, which is a good legal default. But this is also the opposite argument used in other cases like where police are charging citizens with federal wiretapping laws when they are filmed beating a suspect that's handcuffed. Now you want to look at the intent and not the words, again to offer better protection to the citizen.
The problem is we can't have it both ways, so we have to pick (either as a whole, or on a case-by-case basis) whether to follow the letter or the intent. Case-by-case is sloppy and inconsistent, and as-a-whole is itself just as much a problem as going by the letter. I personally prefer "intent". In a democracy it's very rare for a badly-intended law to get on the books, but we're always having problems with badly-worded laws with good intentions getting on the books. It seems to happen continuously and certainly is a problem as laws age. So I conclude that "intent of the law" is the more appealing option. Our legal system with police and courts makes up both facets, the police enforce the letter, and those that pass that filter go to the courts where intent can be applied. Citizens can get out through either door, and so they should only successfully get prosecuted when both letter AND intent pass muster.
I think if you want to make a defense here you're going to have to give up defending the key and look to defending the data. If you can get a court to believe that the data on the hard drive should be considered part of your fourth amendment protected status, you may have a case. If I think to myself "I wish my ex was dead", that's protected. Once I type it into a word document, suddenly my thoughts become searchable and admissible evidence. One can't be used to incriminate me, the other can. If you want to call the document protected, now what happens if I print it out? Is the printout protected too? What if I copy the file? Upload it somewhere? It's a difficult hair to split.
A legal system that requires the assistance of the defendant to prosecute them is clearly going to experience failures. And that's what encryption is currently doing. "Contempt of court" seems to be getting a lot of use nowadays in cases where the law indeed does require the assistance of an uncooperative defendant - it itself is not a problem, but it IS a symptom OF the problem. I'm not against it for the sake of privacy, but I think when it becomes evidence, (and they have jurisdiction) they need access one way or another. This issue has been getting kicked around for quite awhile now and nobody's come up with an easy solution to it yet so I'm not holding my breath.