[hackinthebochs]

I'm pretty sure this isn't true. You cannot prove the hidden volume exists. In fact hidden volumes are completely useless without plausible deniability. The file system of the outer volume will happily overwrite your hidden volume if you tell it to. The point is that you know its there so you intentionally don't write more than 16G on your 20G volume. But the "unused" space looks just like random data so you can't prove there is anything meaningful there.

[Guvante]

Assuming you don't have it protected, you need to be more cautious than that, as the file system is not simply a long stream of bytes.

[hackinthebochs]

Very true, my numbers were just an illustrative example given the context set up by the comment I replied to.

[fuzzbang]

Which you apparently didn't read thoroughly. I clearly state that the secure way to use TrueCrypt is to never mount the hidden volume in protected mode. That will enable the scenario you describe. I even state the reason why you want use it the way I suggest is to minimize the amount of hidden data that is overwritten.