So burn a copy of them which would NOT be looked at until they dot all the “i’s” and cross all the “t’s” with the proper warrant. Then they have what was on his drives when they were returned. They can say they kept an archival copy to be sure everything he had on them was in good order. But if they examine the stuff without a proper warrant, they SHOULD go to jail.
Once law enforcement has gone the wrong way about a particular case, they don’t usually get a second chance at it. They’ve pretty much cooked their goose at this point.