How to disable Due USB power? I need my Arduino Due to work only from external power supply, not from USB power. Is there any way to modify the circuit to allow this? I still need to keep USB communication, but it should happen only when external power is on. Thanks
Disabling USB power is slightly tricky, as the incoming USB power is not only used for potentially powering the system, but it is also used by the various USB interfaces to indicate the presence of a connection.

The best way, looking at the schematics of the Due, would be to remove one or both MOSFETs that switch the USB power - that's T1 for the native port and T2 for the ATMega interface.  That will isolate the USB power from the 5V system while keeping it connected to the VBUS pins of the relevant microcontrollers.