I'm having a bizarre problem with the fat16 library. On a call to SdCard.begin() it throws an error: 2, which the documentation says means the "card did not go ready". This is annoying, but what really perplexes me is that the SdFat library and built-in SD library both work just fine.
I'm using an Arduino Uno with the Adafruit CC3000 Wifi shield. The card is a 4-Gig SanDisk micro SDHC card, formatted with a fat16 filesystem on a 2-Gig partition.