diff --git a/Marlin/src/gcode/gcode_d.cpp b/Marlin/src/gcode/gcode_d.cpp
index 2ebe38bbcf..be431eed0d 100644
--- a/Marlin/src/gcode/gcode_d.cpp
+++ b/Marlin/src/gcode/gcode_d.cpp
@@ -156,20 +156,21 @@ void GcodeSuite::D(const int16_t dcode) {
     } break;
 
     case 5: { // D5 Read / Write onboard Flash
-      #define FLASH_SIZE 1024
+              // This will overwrite program and data, so don't use it.
+      #define ONBOARD_FLASH_SIZE 1024 // 0x400
       uint8_t *pointer = parser.hex_adr_val('A');
       uint16_t len = parser.ushortval('C', 1);
       uintptr_t addr = (uintptr_t)pointer;
-      NOMORE(addr, size_t(FLASH_SIZE - 1));
-      NOMORE(len, FLASH_SIZE - addr);
+      NOMORE(addr, size_t(ONBOARD_FLASH_SIZE - 1));
+      NOMORE(len, ONBOARD_FLASH_SIZE - addr);
       if (parser.seenval('X')) {
         // TODO: Write the hex bytes after the X
         //while (len--) {}
       }
       else {
         //while (len--) {
-        //// TODO: Read bytes from EEPROM
-        //  print_hex_byte(eeprom_read_byte(adr++));
+        //// TODO: Read bytes from FLASH
+        //  print_hex_byte(flash_read_byte(adr++));
         //}
         SERIAL_EOL();
       }