IANAL, but if I understand what I'm reading, here's how it works. (Lawyers, please correct me where I'm wrong):
The 5th amendment protects you from making testimonial statements that would incriminate you. What is testimony, then? It's basically saying something that the prosecutors don't know, or something that isn't self-evidently true. (The police and prosecutors may THINK you robbed the bank, but they can't compel you to admit on the witness stand that you did so, because that would be self-incriminating testimony from you that would clinch the case.)
In this case, however, the prosecution is well aware that the defendant has the information they want: namely, the password to the encrypted drive. They know this because he typed it in previously, in front of ICE agents. Therefore, by providing them the unencrypted contents of the drive, he is not providing new "testimony" - that is, when the defendant reveals that he does indeed know the password, it's nothing new. The prosecution already knows he owns the computer and that he knows how to access the hidden drive. Thus, the 5th amendment can't be used by the defendant to save himself from having to give the contents of the drive to the authorities.
If I'm not mistaken, the authorities can compel a defendant to open a locked safe when they know that person knows where the key is (or what the combination is). I believe the same thing is happening here.
Now, what if hypothetically he had a TrueCrypt hidden container on the drive? And what if the authorities were pretty sure that such a container existed, but couldn't be sure? Could they compel him to testify whether or not there IS a hidden container in the drive? I don't believe so - that would probably tilt the balance into "testimony", which would be protected by the 5th amendment. Ditto in the case of a file called "MYSTUFF.DAT" that the authorities think is probably a TrueCrypt encrypted volume, but can't be sure about. They can't force the defendant to confirm that suspicion.
In this case, the defendant was sunk because of his prior, freely-given revelation that 1) there was an encrypted drive on his PC and 2) he knew how to access it. By giving that information up, he gave up the farm. It's too late to plead the 5th.