Would'nt it be simpler to disconnect USB +5V ?
After all, there is no real need to power the board from USB port.
I think it depends on where you want to go. For me its best not to plug/unplug any devices for FW uploads / CLI setups through USB. My ESC's obviously power the motors through the BEC connection. On pin one I measured PWM outputs at FW upload and at reboot. So without the diode, motor one is spinning up and the USB powersupply breaks down unless I add a second 5V source to the APM.
I had some strange motor behaviour today
One motor would only twich at low throttle settings while the others spin up just fine
Suspecting a fault motor I swapped it for a new one - no change to the 'bad' behaviour
This led me to suspect a bad ESC or bad ESC calibration - I did a couple ESC calibrations - no change
So I did a few run up tests in the back yard - powering up to a low hover - hoping to see what happened
During one run up the 'bad' motor stopped dead and I heard the familiar ESC boot up beeps
After the ESC reboot the motor ran OK (apart from the twitchy behaviour)
So I pulled the thing apart to swap the ESC - damm that power board is buried deep inside!
The one difference with 'bad' ESC when compared to the others is that it is the one supplying the APM through its BEC (in my quad it is the 'back' motor - this is what you get following the wiki constructions instructions)
I'm about to fit an external UBEC and new ESC but it leads me to wonder if some of the problems people are seeing with the same motor burning out more than once and others with un-explained in flight 'flips' could be related to drawing APM and receiver power supply from one BEC on one ESC.
Do we know the start up current from all the bits we put on an Arducopter (APM, Xbees, Sonar, RX etc)?
Do we know that the standard JDrones / 3Drobotics ESC BEC can handle the load without some form of 'brown out' that may 'burn' a motor?
For the people with 'burnt' motors was your motor powered from the ESC that also supplied power to the APM?
First thing to do is use a separate BEC. Be careful, switched BEC can produce high amount of noise, and cause controler crashes or radio noise. They can trig more problems than they will solve.
I would recommand a good quality linear BEC.
Next, opto isolation is a better design rule. This will avoid any ground loop between high power and low power circuits.
So for best results, use an external BEC, and opto isolated ESCs. Then you will never see those problems again, or if you see something similar, it will be simpler to diagnose the source of the problem because you will be able to eliminate the ground loops suspiscion.
Ground loops are terrible problems when low power to high power connections need to be made. Opto coupling is the nice solution. Optos are used everywhere in industrial and commercial products. They are not so much expensive.
The only drawback of opto coupling, is that the LED can fail, then you have to replace the opto coupler. But it is generaly a low propability occurence.