I have an Arduino Mega and the SD card module found at this link: http://www.ebay.com.au/itm/SD-Card-Module-Slot-Socket-Reader-Arduino-ARM-MCU-Read-And-Write-/180868021964#vi-content
I am using this test code to check if it is working: http://arduino.cc/en/Tutorial/CardInfo
The problem was that it was not working, so I started checking voltages on the SD card module. Then I found that when I have the multimeter probes on the SCK (Serial Clock) and GND pins the code works. I unplugged the probes from the multimeter but left the ends attached to the SD card module and it still worked. As soon as I remove one of the probes from the module (either GND or SCK), the code no longer works.
I have no idea what is going on here but is it something to do with the probes changing the resistance of the pins that is making it work?