I am doing a project that uses a high powered servo, and I want to run the Arduino Uno and the servo off of the same 5V power supply but I still want to be able to safely connect my PC to the USB port without risk of back feeding my computer with 5V from the Arduino.
After looking at the schematic for the Arduino, I simply removed used a hot air pen to remove the MOSFET T1 on the Arduino Uno which allows USB power to feed the +5V net. The only downside to this is that I need to have my external 5V power supply on in order to program the Arduino with the PC. I marked my board as modified and then taped the MOSFET to the board so I can reinstall it later if I want to. The MOSFET is a SOT-23 packaged part, so it is pretty easy to remove.
I first tried removing the polyfuse, but this prevented the USB from connecting since the USBVCC is used by the USB microcontroller so that did not work.
I also considered changing the threshold of the comparator so it would turn off the MOSFET when 5V was applied and it back feeds to Vin. I was going to do this by changing one of the 10k resistors to a different value, but this was impractical since they are arrays and you can't change only one resistor.
Removing T1 was a simple fix that will keep my PC safe when back feeding 5V.
If you wan to add reverse polarity protection without the higher voltage drop of either a PN diode like the M7 diode (~0.7V) or a good schottky (~0.5V), you can always play the trick of using a backwards connected P-channel mosfet. See this article:
This circuit is simple enough to add to the breadboard area of many boards, so if this is important to you, it's worth considering. I have used this trick in some of the boards I design at work.
Best of luck, and I hope this post gives some good ideas !