You supply it, of course. But having the drive encrypted buys you time. The cops have to take the machines into evidence, nominally send them out to the lab f

You supply it, of course. But having the drive encrypted buys you time. The cops have to take the machines into evidence, nominally send them out to the lab for processing, and only then do they run into your encryption key. They then need to go back to the judge, and get a court order to compel you to supply your encryption key. By which time you've gotten a lawyer, and can fight back.