While updating to AC2.7 the 2560 Bootloader got corrupted and would not work.
I have tried to flash the Arduino Bootloader over the ICSP but have too many errors.
Alot of fustrating time later I have to give up. I tried different Programers , usbasp, usptiny, arduino as Isp. Writing bootloader gets verification error.
I can change fuse to No Bootrst and load a blink the Led program over ICSP and it runs ok but can not load the whole Arducopter Hex over the ISP without error.
I have come to the conclusion that the SPI connections to the sensors is interfering with the ICSP function and that the 2560 can not be recovered .