On the Mega, PB0, 53, must be set to an output even if not used for SS. That tells the Mega that is SPI master. If left as an input and it goes low, the Mega goes into SPI slave mode.
On the Mega, PB0, 53, must be set to an output even if not used for SS. That tells the Mega that is SPI master. If left as an input and it goes low, the Mega goes into SPI slave mode.