Re: Device with multiple partitions
Bitlocker doesn't do this - it encrypts but doesn't conceal you have done so. However, there are several successors to TrueCrypt such as BestCrypt and VeraCrypt which do support Hidden Volumes / Hidden Containers which are what you're referring to.
Because all of an encrypted partition or file appears as random noise, there's theoretically no way to distinguish empty space on the disk from used space. So you can have two encrypted containers appearing as one and determine which you're accessing by the password. Think of it as a magic door. You knock three times and it opens on a room where you've stored a few innocuous things like your email password. Knock five times and it opens on a room where you hide the state secrets you just stole. The magic is that the number of knocks can't be guessed so you just tell the interrogator it's three knocks and that's the only room they'll ever see and they can't prove that a different sequence of knocks would show a different one. It adds the last vital component of encryption which is deniability.