[syrrim]

You can prove you haven't used deniable encryption. Encrypted data looks random. As long as your disks and files don't contain unexplainable random sections, there must be no encrypted data there. Steganography might have been used to hide the encrypted data in otherwise meaningful data, but that is a separate concern.

[fsflover]

Truecrypt hidden volume looks like free space without the right password. You can't prove you don't have data there.

[zapt02]

That's not strictly true, you can format a drive to contain all zeroes. That can then be demonstrated easily via a hex editor looking at the drive. Truecrypt volumes will always look like random scrambled data (like if you formatted your drive with a "secure erase" method).

[Sebb767]

Allthough this presumes that the guy with the wrench takes the time to check out your disk in a hex editor and that he finds the statement that you directly zero everything you delete to prepare for exactly this scenario not at all suspicious. Doesn't sound like a great plan to me.