Cannot access SD card on WINC1500
Do I need to pass a parameter to SD.begin()?


Yes, you need to pass the Chip-Select (CS/SS) pin number. What number depends on how you have it hooked up, the default is 10. (yours might be 4)

n.b. according to the docs a standard arduino can't use the WiFi, and the SD at the same time; presumably the mega has enough memory.