Generally speaking, any non-testimonial evidence entered into court must be backed by a witness testifying to the nature, content (e.g. literally reciting the incriminating parts), and origins of the evidence. And that person has to have had a proper relationship to the evidence to do so, which is usually the person with the most direct and close relationship to an item or act. For example, if you want to enter verbal statements into evidence, the best and proper person to testify to them is usually the person who made the statement, not a listener.
Another way to look at it is that the only real evidence in court is witness testimony; everything else is just window dressing. Thus we have the Fifth Amendment: "[no person] shall be compelled in any criminal case to be a witness against himself."
How does that get turned into a rule that a defendant can't be forced to divulge information outside court? Because to be admissible in court such information would normally require the defendant to be a witness. If it's not admissible then there's no legitimate basis for the state to compel an act. That leads to two corollaries: 1) if the person isn't a defendant, or is given immunity, he would never be required to be a witness against himself (just a witness against someone else), and thus the Fifth Amendment isn't implicated; 2) if the evidence obtained from the information would be admissible without the defendant's testimony, the Fifth Amendment likewise isn't implicated.
#2 is the case here. Because the government can already show ownership of the hard drives through other witness testimony (e.g. testimony of a relative, purchase receipts, etc), it would be categorically unnecessary for the defendant to testify in court to the incriminating act of divulging the password. However, if for some reason it later turned out that the government couldn't show ownership independently, then of course the hard drives would be inadmissible; not because of the forced password disclosure, per se, but simply because the defendant himself couldn't be compelled to be a witness to his act of divulging the password--an investigator couldn't testify that the defendant disclosed the password as that would be hearsay.[1] But such a turnaround is rare as a court won't compel disclosure unless it's clear there's ample independent and admissible evidence of ownership.
[1] There are lots of exclusions and exceptions to hearsay, especially regarding defendant admissions, but they're disallowed if they would effectively nullify the spirit of the Fifth Amendment if permitted for compelled acts. If you're a strict textualist, as most conservative-leaning people believe themselves to be, then one would presumably be okay with permitting those exceptions, particularly those exceptions which existed at the time of ratification of the Fifth Amendment.