And if you leave interrupts disabled, please understand Arduino can't cause Teensy to go into programming mode, because you've disabled the USB interrupt. You will need to press the button on your Teensy to get it to upload code. After several seconds of trying, the software will advise you to press the button, because it couldn't communicate. Don't mistake that message for a dire error. It's perfectly normal when you've done something on your Teensy that inhibits USB communication.