card.transfer_port => transfer_port_index

Marlin/src/gcode/queue.cpp

         stream_state = StreamState::PACKET_TIMEOUT;
         return false;
       }
-      if (!serial_data_available(card.transfer_port)) return false;
-      data = read_serial(card.transfer_port);
+      if (!serial_data_available(card.transfer_port_index)) return false;
+      data = read_serial(card.transfer_port_index);
       packet.timeout = millis() + STREAM_MAX_WAIT;
       return true;
     }
       millis_t transfer_timeout = millis() + RX_TIMESLICE;
 
       #if ENABLED(SDSUPPORT)
-        PORT_REDIRECT(card.transfer_port);
+        PORT_REDIRECT(card.transfer_port_index);
       #endif
 
       while (PENDING(millis(), transfer_timeout)) {
               stream_state = StreamState::PACKET_RESEND;
               break;
             }
-            if (!serial_data_available(card.transfer_port)) break;
-            read_serial(card.transfer_port); // throw away data
+            if (!serial_data_available(card.transfer_port_index)) break;
+            read_serial(card.transfer_port_index); // throw away data
             packet.timeout = millis() + STREAM_MAX_WAIT;
             break;
           case StreamState::PACKET_TIMEOUT:
        * receive buffer (which limits the packet size to MAX_CMD_SIZE).
        * The receive buffer also limits the packet size for reliable transmission.
        */
-      binaryStream.receive(serial_line_buffer[card.transfer_port]);
+      binaryStream.receive(serial_line_buffer[card.transfer_port_index]);
       return;
     }
   #endif

Marlin/src/gcode/sdcard/M20-M30_M32-M34_M524_M928.cpp

       SERIAL_ECHOLN(p);
       card.openFile(p, false);
       #if NUM_SERIAL > 1
-        card.transfer_port = command_queue_port[cmd_queue_index_r];
+        card.transfer_port_index = command_queue_port[cmd_queue_index_r];
       #endif
     }
     else

Marlin/src/sd/cardreader.cpp

 int8_t CardReader::autostart_index;
 
 #if ENABLED(BINARY_FILE_TRANSFER) && NUM_SERIAL > 1
-  int8_t CardReader::transfer_port;
+  int8_t CardReader::transfer_port_index;
 #endif
 
 // private:

Marlin/src/sd/cardreader.h

 
   #if ENABLED(BINARY_FILE_TRANSFER)
     #if NUM_SERIAL > 1
-      static int8_t transfer_port;
+      static int8_t transfer_port_index;
     #else
-      static constexpr int8_t transfer_port = 0;
+      static constexpr int8_t transfer_port_index = 0;
     #endif
   #endif