If i try to upload a sketch to my due via the programming port, the IDE writes "no device found on COM18". But it has already worked. I use windows and I have installed the driver for the board. However when I try to upload a sketch via the native usb port it works. I have also tried it with another pc but there was no difference. How can I fix this problem.
Try connecting using the native usb port. Then press the erase and the reset button at the same time. If everything went well, the bossac port will connect to your pc. You can upload a random sketch with the arduino ide then. After that, press the reset button again. And everything should work again.