Slippery Slope
The court must make the prosecution prove that the individual not only has access to the files in question, but also has 1. The authority to access the device in question, 2. The ability to access that device with a key, or other form of either digital or physical security token that prevents prosecutors from accessing that information, and 3. Has accessed the device in a directly related matter to the case at hand.
Basically, it should fall on the prosecution to prove that they must have access to the device, and that the individual in question has not only the ability to access the device, but the authority to access that device as well. For example, you couldn't arrest an IT Admin at a company, and ask him to divulge the encryption keys on all of the network computers, since he's not authorized to do such a thing.
The defendant should not have a case for "I don't know the password to the hard drive." before he is ordered to unlock the hard drive.
Also, as another item, the contents of a hard drive should fall under separate pieces of evidence, and should also follow the rules of appropriate search and seizure, requiring a warrant, and documenting the contents thereof. If the case is for child pron, and the search turns up nothing of the sorts, but it turns out he was torrenting the crap out of Walt Disney, the poor fellow shouldn't be liable to be charged with Copyright Infringement instead.