This is the question I've been presented with, too.
Currently, I have a Catalex microSD Card module that interfaces beautifully with my Arduino Nano v3 over SPI. However, my users have shown me that it'd be too much of a pain to disconnect my device and remove the cover to access the SD card so they can plug it into the computer.
Ideally, they want a way to easily interface with the PC/Mac; USB is preferred.
I can't seem to find a decent explanation or tutorial to point me in the right direction. I'd love to just solder a USB-B header to my board, and couple resistors/ICs so that the PC/Mac can interface with the SD card when the device is off, and the Arduino reads the SD card when the device is on.
(If I have to design my own SD card setup to do have the SD socket and additional ICs to interface with both Arduino and computer, that'd be fine, too.)
If any information could be provided to at least point me in the right direction, I'd be greatly appreciate.
Thanks in advance,
Kyle