It really depends. If the police have some corroborating evidence that you have records of all your illegal arms dealing (or whatever) on your encrypted drive

It really depends. If the police have some corroborating evidence that you have records of all your illegal arms dealing (or whatever) on your encrypted drive then the court can still compel you to decrypt it. At that point you can decrypt it or refuse/feign that you've forgotten (risking of contempt of court).