IANAL, but the 5th circuit appeals court has ruled that the 5th amendment means that a person cannot be forced to give up the combination to a lock. It is informational, and it is testimony.

Any discussion of a physical key is without merit. A physical key, is not the same as a "encryption key". For the purpose of law, an "encryption key" is the same as a combination to a safe.

There's actually a ton of precedents on this. Google 5th amendment and combination lock.

The reason it is testimony is because if you provide a password and it works then you've just proven that the contents are yours: you have incriminated yourself. If the prosecution can prove that the contents are yours without the password, then at that point you can be forced to give up the password.

Takeway: your encrypted drive had better have a different password than your login, and you'd better be able to deny that it is yours.

EFF will win.