Thank you for your quick response, I've wired it like the schematics shown on this topic http://arduino.cc/forum/index.php/topic,8863.0.html , I ripped the card holder from a multiple card-to-USB reader, and maybe it doesn't have enough power because the arduino 3.3V is also connected to an RTC (chronodot that uses few mA, 200µA according to specifications), could this be the problem?