Again, they know the book, the book is entitled "The encryption code for the persons computer" they know exactly what the book is, what they don't know is the contents of that book.
I know what encryption is, it is a safe to protect your belongings so that even if it is stolen, others can not open it to use it. I understand exactly what it is. They have the safe, they can't open it though. Now, if he actually started writing texts in code in that safe, then you might have something, but he wasn't pulling a Nostradamus. He literally took his contents and put it behind a combination lock. That is what the encryption is.
You mean unless you can prove that more than just him had access to that drive is what you mean. But the rest just sounds weird. If someone else had an encrypted drive before makes no difference because the ownership of THIS drive they already know of and is confirmed. And the fact it is HIS encryption again has already been confirmed. So him giving them the key is not self incrimination logically. (Even though the courts have bad verdicts on it as others have claimed because they don't know what they are talking about).
The encryption IS the safe and just putting the safe "On a computer" like a bunch of stupid patents, doesn't change that fact that it is a digital safe.