Okay - but what if the argument is literally "I can't - the keyfile is lost"

or

Ignoring the details of this case, what if the court is ordering you to decrypt something that isn't even encrypted? It's just random data? They assume it is. You assume it isn't.

I suppose in this case, the fact that the someone of authority already saw unencrypted contents gives them enough reason to be confident the drive is encrypted and the defendant has the ability to decrypt it.