connection between RasPi & Arduino Mega

Dear Community, i´ve got a problem with the connetcion of the RasPi and my Arduino Mega, i can see a ttyUSB* in the DEV-direction of debian. Sometimes i get a ACM0, when i do nit get it, i have to reinstall debian. And after some restarts of the system i miss it. The Arduino has his own energy support. When i build the system with my arduino uno, i ever get a ttyUSB*-device. The UNO does not support my GPS and my weather meter together. Maybe some of you can help me.Thanks a lot.