May be there is an error on wiring mega.
As I read specs of SPI of nano and mega we can discover that :
MEGA :
SPI: 50 (MISO), 51 (MOSI), 52 (SCK), 53 (SS). These pins support SPI communication using the SPI library. The SPI pins are also broken out on the ICSP header, which is physically compatible with the Uno, Duemilanove and Diecimila.
NANO:
SPI: 10 (SS), 11 (MOSI), 12 (MISO), 13 (SCK). These pins support SPI communication, which, although provided by the underlying hardware, is not currently included in the Arduino language.
So, use the PIN 53 instead of PIN10 with the Mega board.
May be it will works as SS means ship select !