I believe in the US you can still be legally compelled to give up the encryption key(s), and simply held in contempt (indefinitely?) until you do.
So when they find a drive that passes the Chi^2 randomness test they know there has to be encrypted data on there, so you give them the key and they decrypt the outer ring (which contains nothing important). At that point they're stuck. They suspect there's got to be more to it, but they can't point to anything and say "decrypt this!", because there isn't anything to point to. So technically, they have to let you go. Or so I heard.
I remember hearing about this being used as a justification by a judge in a case relating to encryption. It's just sad that most cases involving encryption / computer crimes are related to child porn charges so people are more worried about punishing the person than they are about protecting the rights of everyone that is brought into the court system.