That's the whole point of this feature as far as I can tell: It should be impossible (ignoring flaws in the software/system) to know if there's a hidden volume.

That's the whole point of this feature as far as I can tell: It should be impossible (ignoring flaws in the software/system) to know if there's a hidden volume. You give the key to the decoy, smile and ask if you can comply in any other way and move along. The english phrase that I see connected to this all the time seems to be plausible deniability [1]? Since hidden volumes are an optional feature and - again, in theory - cannot be proven to exist, you can always claim that this is the only password. Even if the judge/attorney knows a thing or two about true crypt. 1: https://secure.wikimedia.org/wikipedia/en/wiki/Plausible_deniability https://secure.wikimedia.org/wikipedia/en/wiki/Plausible_den...