[function_seven]

That's the beauty of full-disk encryption. Even the empty space is encrypted. So the hidden volume is truly hidden. Even TrueCrypt has no idea the hidden volume exists if you unlock the outer volume with a different key.

Truly empty-space is indistinguishable from a secret inner volume.

[tomflack]

Why can't I write a program that tries to expand itself to use any available space, then runs in to a wall if the "empty space" is actually encrypted data? If the space used by data + my program adds up to less than the total capacity of the disk, it indicates something is hiding right?

[ehsanu1]

You seem to be misunderstanding. Read your parent's last line again. "Empty space" is indistinguishable from encrypted data. On the hard disk, everything will just look like randomized bits, empty space and data alike. There is no way to write the program you propose without the encryption key(s). So there's no way to tell, unless you have all the keys.

[tomflack]

praptak's reply above yours explained exactly what I needed explained. The encrypted data will just be written over.

[praptak]

The program will just overwrite the data of the hidden volume. That's why it's important to have a lot of empty ("empty") space on the primary volume when you have a hidden volume there.