Earlier quoted context omitted.
This is the way things should work. But, there was a widely reported case recently where a man accused of possessing child pornography has been held in jail for a long time [0]. From what I can tell, he's been in jail for contempt of court for nearly 2 and a half years! [0] https://arstechnica.com/tech-policy/2017/03/man-jailed-indef...
That's because the issue is more complicated than any absolute statements. 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?