By the way, in case somebody doesn't understand what the 'fifth' is, it's the lack of authority by the government to force somebody to testify against themselves.
To understand its roots, you have to look back at when kings and other rulers would capture and torture somebody to get a 'confession'. When people are tortured, most will confess to just about anything, so torturing is a very simple way to get a conviction (or to murder somebody, whichever comes first) and using torture to get a conviction can often lead to murder at the end of torture anyway.
But that is the origin, when somebody says: "I take the fifth", what they mean is that they will not testify against themselves. But to testify against yourself you have to be a suspect, you have to be the one on trial, that's why Lois Lerner, the IRS director "taking the fifth" makes no sense, she wasn't on trial.
Saying: 'I am taking the fifth' only makes sense when you are on trial or a suspect of a police investigation, but it doesn't make sense to say "fifth" when you are testifying to Congress.
You can refuse to answer questions, but invoking the fifth amendment has no meaning in that context, AFAIC she admits her guilt and/or lack of understanding of the law when she says that.
OTOH in this case I am NOT convinced that the fifth amendment is relevant in cases of encrypted data!
Forcing somebody to unlock their data is not the same as forcing somebody to sign a statement. After all, it's real data, it's already there. By being forced to unlock the data you are not being forced to say something new, it's not new information that is on the disk, it's not like you are forced to say: I am guilty, here is the body.
You are forced to open a box that may have data providing that you are guilty, but that information is already there and it's not new, you weren't forced to first create that data and then give it up, you are forced to open the data that existed already in a form that is not attached to you, it's independent of you, it is already existing outside of you.
How is that equivalent of being tortured (or punished) into saying the words: I am guilty, here is the stuff you are looking for?
I am just being pedantic here, the fifth is not necessarily a protection against being forced to give up data that already exists that you do not have to create or produce at the moment of giving it up.
I disagree with your take on the "fifth", you should be able to take the fifth on any statement that COULD be used against you in court, just because you are not there now, doesn't mean that the statement you make will not land you there.
in addition, i will also take the counter point on decrypting the HD. i take the essential meaning of the fifth as to you should never have to provide evidence against yourself. By providing them with the password, or whatever the key is, to the encrypted HD, you are providing them with evidence that they would not be able to get with out your assistance. If they are able to get the evidence(brute force decryption, or what have you) without your assistance, with a warrant, that is fine, but you should never have to assist, in any way, in your own prosecution.