✨ M21 P / S / U - Select Volume (#23780)

Co-authored-by: Scott Lahteine <thinkyhead@users.noreply.github.com>

Marlin/src/gcode/sd/M21_M22.cpp

@@ -29,8 +29,21 @@
 
 /**
  * M21: Init SD Card
+ *
+ * With MULTI_VOLUME:
+ *  P0 or S - Change to the SD Card and mount it
+ *  P1 or U - Change to the USB Drive and mount it
  */
-void GcodeSuite::M21() { card.mount(); }
+void GcodeSuite::M21() {
+  #if ENABLED(MULTI_VOLUME)
+    const int8_t vol = parser.intval('P', -1);
+    if (vol == 0 || parser.seen_test('S'))       // "S" for SD Card
+      card.changeMedia(&card.media_driver_sdcard);
+    else if (vol == 1 || parser.seen_test('U'))  // "U" for USB
+      card.changeMedia(&card.media_driver_usbFlash);
+  #endif
+  card.mount();
+}
 
 /**
  * M22: Release SD Card