teensy-sdfat/MultipleCards.txt

SdFat has support for multiple SD cards.  This requires multiple instances
of SdFat objects.

You must edit SdFatConfig.h to enable multiple instances of SdFat. Set
USE_MULTIPLE_CARDS nonzero like this:

#define USE_MULTIPLE_CARDS 1

Look at TwoCards.pde in the SdFat/examples folder. This example demonstrates
use of two SD cards.

Read WorkingDirectory.txt for more information on volume working
directories and the current working directory.