NO_SD_DETECT option (#20741)

Marlin/Configuration_adv.h

@@ -1388,13 +1388,16 @@
    * Set this option to one of the following (or the board's defaults apply):
    *
    *           LCD - Use the SD drive in the external LCD controller.
-   *       ONBOARD - Use the SD drive on the control board. (No SD_DETECT_PIN. M21 to init.)
+   *       ONBOARD - Use the SD drive on the control board.
    *  CUSTOM_CABLE - Use a custom cable to access the SD (as defined in a pins file).
    *
    * :[ 'LCD', 'ONBOARD', 'CUSTOM_CABLE' ]
    */
   //#define SDCARD_CONNECTION LCD
 
+  // Enable if SD detect is rendered useless (e.g., by using an SD extender)
+  //#define NO_SD_DETECT
+
 #endif // SDSUPPORT
 
 /**

Marlin/src/inc/Conditionals_post.h

@@ -323,6 +323,11 @@
  */
 #if ENABLED(SDSUPPORT)
 
+  // Extender cable doesn't support SD_DETECT_PIN
+  #if ENABLED(NO_SD_DETECT)
+    #undef SD_DETECT_PIN
+  #endif
+
   #if HAS_SD_HOST_DRIVE && SD_CONNECTION_IS(ONBOARD)
     //
     // The external SD card is not used. Hardware SPI is used to access the card.