So if they force disclosure of a password and the data that was "known" to be on there isn't there, do you get to sue for violation of your fifth amendment rights?
Well, the more important question is: what about things on there that they didn't know about beforehand?
In other words, say they know this guy has documentation on mortgage fraud - ok, legally if they know he has specific documentation on there maybe they can force him to unlock it. But, for the sake of argument, say this involves the Russian mob and part of what is on the hard drive would implicate him in an as-yet unsolved (or uninvestigated as "natural causes/accidental") murder? If they compel him to unlock the drive for the "financial data" and they find proof of his complicity in a murder, is that grounds to charge him with conspiracy to commit murder, a charge they wouldn't have had any knowledge or proof of before then? Or, what if they find kiddie porn on it, can they then charge him with kiddie porn trafficing? Or is that then "fruit from the poisonous tree" gained illegally and not able to be used against him?
The easy solution here, of course, is a court judgment that he cannot be charged with any new crimes based on what it revealed on the hard drive, or even simpler - finding him "guilty" based on the evidence you claim is already enough to prove he is guilty, and then offering him leniency on sentencing for those charges if he unlocks the drive to allow future investigations into others involved (or "turning him into an informant" basically).