You do realize that encrypting your hard drive with a key you keep in your head really is just security through obscurity
No it's not. It involves an obscure key, sure. But that's not what is generally meant by the phrase "security through obscurity". What is meant by that phrase is a system that is secure only as long as the system's implementation is kept secret. In the case of password protection, the implementation is known, it's just the key that is secret. In security through obscurity, if somebody discovers the implementation of the system, they can get in.
An encryption system that relies on security through obscurity would be one that, if you got hold of the source code, you could decrypt anything encrypted with it, key or no key.