They didn't find Air France 447's black boxes until more than 2 years after it disappeared, after the beacon batteries died on the flight data recorder (FDR) and cockpit voice recorder (CVR), and when they did find it, there was usable data that helped determine the cause of the crash. They knew where it crashed though, as they quickly recovered large pieces initially, while with MH370 they still don't know if the southern IO is the right place they are looking.
