The begin() function calls card.init() with SPI_HALF_SPEED:
boolean SDClass::begin(uint8_t csPin) {
/*
Performs the initialisation required by the sdfatlib library.
Return true if initialization succeeds, false otherwise.
*/
return card.init(SPI_HALF_SPEED, csPin) &&
volume.init(card) &&
root.openRoot(volume);
}
Which means it is running too fast. In Windows, if you open the following file (it is slightly different on Mac):
/libraries/SD/SD.cpp
You will find the begin() function declaration, just change the SPI_HALF_SPEED to 2.
As for the TXB0104, there are breakout boards available ready soldered:
8-channel Bi-directional Logic Level Converter [TXB0108] : ID 395 : $7.95 : Adafruit Industries, Unique & fun DIY electronics and kits (This is actually the 8 channel version, TXB0108)
http://www.nanocore12.com/products/details/27/4/accessories/breakout-board,-level-shifter,-3v/5v,-14-pin-wide-dip.html (4 channel TXB0104)