Hi,
is it so hard to believe that your board is running on 5V and not on 3.3V even that #*&! website says something different? That you measured the 3.3V line at 4.2V is a sign that your SD card is fighting a deathly battle against the 5V. What happens is the following: The SD card is connected to 3.3V at VCC, but at 5V at one of the other pins. An internal protection diode in the card tries to clamp the excess voltage at the pin with 5V to the 3.3V supply, but that supply is also weak because the crapduino uses some 3.3V from the USB-seriell-converter chip instead of a powerful extra regulator. This way the 3.3V regulator looses the battle and its output voltage rises to 5V minus the typical diode foreward-voltage of 0.7-0.8V = 4.2V. This is not how it should be done! You can destroy your AVR, the USB-Seriell-Converter or the SD card. I don't know how long this battle can take place and who will win, but you might have already broken something. Hint: Try if your SD card still works within a PC before spending more time frying stuff...
Believe it: YOU NEED LEVEL-SHIFTERS or a SD-shield that has those!