I am using a Teensy 4.1 in my project. When I plug the Teensy into my laptop (running Linux), no port named something like ttyACM0 is created in the /dev/ directory. When the Teensy is connected to my laptop and I press the PROGRAM MODE button, the red LED turns on (still no port in /dev/). But, when I plug the Teensy into an external supply like a charging brick and press the PROGRAM MODE button, the red LED blinks. Any idea what this could mean?