CrossRoads:
I think you'll need to connect to pins 19-24 for 3.3V, Gnd, CS, SCK, MISO, MOSI to access the SD card.
I tried that this morning, but no success so far.
I tried moving the four data lines from my SD-Card holder to the four data pins on the FN-RM01 mp3 module. I connected MOSI to SD_CMD, thinking this must be the input to give commands to the card. That left MISO to be connected to SD_DAT. (CS and SCk connections were obvious.) I passed all four lines through a bilateral voltage shifter using a 3.3v regulator to set the low side.
It didn't work.
So I searched Google to see if my connections could be wrong, and found a page that says:
CMD, carrying commands from the host and responses from the card
DAT, carrying data from the host or data from the card
So my standard SPI connection had no chance of working, since MOSI & MISO each send data in one direction, while CMD & DAT each send data in both directions.
Experimenting a little further, (1) I found neither the Arduino nor the mp3 module could read the card while these data lines were attached. (2) Even with only the separate card reader and mp3 module attached together, the mp3 module could not read the card, whether the card was placed in the separate card reader, or in the module itself. (3) I concluded from 1 & 2 that the mp3 module does not do any switching; instead, all three items (module, SD Card, and the data pins must be hardwired together.
So now I believe I must use an external SD-Card holder, and switch the card holder's connections between Arduino and mp3 module. I even got an NTE40116B quad bilateral switch ready for the purpose; but didn't try it, having discovered the CMD/DAT and MOSI/MISO protocols don't match.
I guess the next question is: What do I need to do to reconcile the differences between Arduino's MOSI/MISO, and the mp3 module's CMD/DAT data connections to the SD Card? (Or must I just give up, and force people to move the SD Card back and forth between two SD Card readers?