teensy-sdfat/SPI_Transactions.txt

To enable support for SPI transactions, edit SfFatCinfig.h and modify these
defines.

//------------------------------------------------------------------------------
/**
 * Set ENABLE_SPI_TRANSACTION nonzero to enable the SPI transaction feature
 * of the standard Arduino SPI library.  You must include SPI.h in your
 * sketches when ENABLE_SPI_TRANSACTION is nonzero.
 */
#define ENABLE_SPI_TRANSACTION 0
//------------------------------------------------------------------------------
/**
 * Set ENABLE_SPI_YIELD nonzero to enable release of the SPI bus during
 * SD card busy waits.  
 *
 * This will allow interrupt routines to access the SPI bus if 
 * ENABLE_SPI_TRANSACTION is nonzero.
 * 
 * Setting ENABLE_SPI_YIELD will introduce some extra overhead and will
 * slightly slow transfer rates.  A few older SD cards may fail when 
 * ENABLE_SPI_YIELD is nonzero.
 */
#define ENABLE_SPI_YIELD 0