I remember watching a def con (probably) presentation where they tried putting explosives in their drives to destroy them at a moment's notice rather than letting them get seized
it did not work very well as I recall
nostr:npub15fkerqqyp9mlh7n8xd6d5k9s27etuvaarvnp2vqed83dw9c603pqs5j9gr I am not sure about encryption headers in files but I know that's common for FDE because the thinking is it just doesn't matter if you have a strong password. Deniable is for when you're up against a government and they'll just throw you in jail until you give them the password if they can prove there's a petition.
They don't know the length. They just try all combinations up to a certain point.
Discussion
nostr:npub15fkerqqyp9mlh7n8xd6d5k9s27etuvaarvnp2vqed83dw9c603pqs5j9gr encryptions gotten pretty fancy. Apparently there's such thing as an encrypted operating system now. Beyond me how it works.