Remember the 5th Amendment applies to passwords. No law enforcement officer may EVER compel you to log in to a device with a password / pass code / unlock pattern, for any reason.
[0] https://arstechnica.com/tech-policy/2017/03/man-jailed-indef...
One case decided that law enforcement can compel you to turn over documents that it knows you have possession of, regardless of where they exist. You're not testifying against yourself; you're turning over evidence.
Another case has decided that they can't compel you to turn over a password so they can search for evidence.
So, it's not incriminating yourself to decrypt something everyone already knows you have. That's kinda weird and concerning, depending on how they proved the hard drive's contents without decrypting it. I mean, if it's just someone saying "I saw it on his computer" then that's problematic in my opinion. Whereas if they have server logs, or logs from a different hard drive of his detailing the contents of the encrypted drive, or something more concrete than hearsay, I don't see an issue.
Edit: I guess the issue is: why do they need the contents of the drive if they can already prove the contents of the drive enough to convict?
The only way to positively know that one has possession of the documents in question is to read them. If they need someone's help to decrypt the documents, then they at best have a very strong suspicion/circumstantial evidence of possession.
Even if they intercepted transmission of the documents in question, they don't know if the documents are still in possession or have been deleted until they've gotten the suspect to cooperate and decrypt all of the suspected documents.
If one had a hidden encrypted partition filled only with copies of the U.S. bill of rights, and the government used sworn testimony of definite knowledge that the hidden partition contained contraband in order to force disclosure of decryption keys, what's the likelihood that a good lawyer could get enough damages out of the government to make it worthwhile to sit in detention for a month or two while things got sorted out?
How about this theoretical situation: I place an encrypted file in a hard drive belonging to someone I dislike, and then frame them circumstantially with the likely possession of contraband data on said hard drive. Since he owns the device, he may be so unlucky as to have the authorities believe he knows how to decrypt the file despite him claiming he has no knowledge of how the file came to be or how to decrypt it. If they will not accept that argument, this seems like a good way to place a political opponent in jail indefinitely by someone with the means to craft such a scenario
Here: https://blogs.wsj.com/digits/2014/10/31/judge-rules-suspect-...
Unsure about federal application, actually. We as country need to get our shit together regarding device privacy. You should need a warrant to look at a phone, laptop, whatever, with or without a password. The security of the device should be meaningless.
As for me, I'm grateful that I have the resources to push back should this ever happen to me, jailtime or included. That's a luxury probably very few Americans can realistically afford (seeing as they can just throw you in jail for 48 hours).
I believe the rationale was that they can't ask you to incriminate yourself (give up your passcode), but you don't have to say anything for them to force you to give your fingerprint.
That said, you shouldn't ever rely solely on biometrics. In addition to the legal reason, an adversary could also unlock your phone if you were unconscious / etc using your thumb.