Adding USB MSD implementation to the HAL: Now, when you plug the Native USB port of the Arduino DUE to your PC, it will be detected as a composite USB device: One of the devices is a USB CDC (Serial port over USB) with native USB handshake (no more overflows!!) (set SERIAL_PORT to -1 to use it as main serial port) and also, a MSD (USB Mass Storage Device) will be detected, and the SD card present on the socket will be reflected as a Removable disk on your PC)

Marlin/src/HAL/HAL_DUE/HAL_Due.h

@@ -40,15 +40,13 @@
 //
 // Defines
 //
-
-#if SERIAL_PORT == -1
-  #define MYSERIAL SerialUSB
-#elif SERIAL_PORT >= 0 && SERIAL_PORT <= 4
+#if SERIAL_PORT >= -1 && SERIAL_PORT <= 4
   #define MYSERIAL customizedSerial
 #endif
 
 // We need the previous define before the include, or compilation bombs...
 #include "MarlinSerial_Due.h"
+#include "MarlinSerialUSB_Due.h"
 
 #ifndef analogInputToDigitalPin
   #define analogInputToDigitalPin(p) ((p < 12u) ? (p) + 54u : -1)
@@ -153,4 +151,16 @@ void HAL_enable_AdcFreerun(void);
 #define GET_PIN_MAP_INDEX(pin) pin
 #define PARSED_PIN_INDEX(code, dval) parser.intval(code, dval)
 
+// Enable hooks into idle and setup for USB stack
+#define HAL_IDLETASK 1
+#define HAL_INIT 1
+#ifdef __cplusplus
+extern "C" {
+#endif
+void HAL_idletask(void);
+void HAL_init(void);
+#ifdef __cplusplus
+}
+#endif
+
 #endif // _HAL_DUE_H