Together, the two data-collecting devices can paint a picture nearly impossible to piece together through other means, which is one reason that Razak, in listing his priorities for the investigation into the downing of Flight 17, ordered them as: “first is the bodies, second is the black box, and third is the crash site.”
Sounds a bit optimistic for me. I do not expect more than a sudden interruption of data from a flight which was perfectly normal up to that point. Flight path was already known from several radar sources.
Tampering with any digital data of known format is a piece of cake. As thousands of people around the world must be able to read out Black Boxes, nothing about their way of working or the data format is secret. But I seriously doubt that such action was necessary, there is nothing and has never been any data on the recorder after the missile destroyed the Cockpit and ripped the nose section including the avionics bay off the plane.