card.init(SPI_FULL_SPEED,SD_CS_PIN) returns false for Arduino Mega
53 is a SS pin if the Mega is SPI slave. Pin 10 is connected to shield to select it as a listening slave. So let the parameter set to pin 10. Additionally on Mega set pin 53 to output, otherwise the Mega can 'fall' into SPI slave mode.

SPI pins 50, 51 and 52 are connected to the ICSP header too. The 6 pin ICSP header is on the same place relative to Uno an Mega common pin headers.