Default to 24 MHz clock on Teensy 3.x

utility/Sd2Card.cpp

 
 static void spiInit(uint8_t spiRate) {
   switch (spiRate) {
-    case 0:  settings = SPISettings(12000000, MSBFIRST, SPI_MODE0); break;
-    case 1:  settings = SPISettings(8000000, MSBFIRST, SPI_MODE0); break;
-    case 2:  settings = SPISettings(6000000, MSBFIRST, SPI_MODE0); break;
+    // the top 2 speeds are set to 24 MHz, for the SD library defaults
+    case 0:  settings = SPISettings(24000000, MSBFIRST, SPI_MODE0); break;
+    case 1:  settings = SPISettings(24000000, MSBFIRST, SPI_MODE0); break;
+    case 2:  settings = SPISettings(8000000, MSBFIRST, SPI_MODE0); break;
     case 3:  settings = SPISettings(4000000, MSBFIRST, SPI_MODE0); break;
     case 4:  settings = SPISettings(3000000, MSBFIRST, SPI_MODE0); break;
     case 5:  settings = SPISettings(2000000, MSBFIRST, SPI_MODE0); break;