I have an arduino installed in a system that provides +12V directly to the power jack that is soldered to the board, not using a plug. This works fine as long as no usb plug is installed. As soon as I plug in a USB plug the computer fails to recognize the UNO. If I unplug the soldered 12v connection, then the uno is recognized by a computer.
I am trying to have this UNO power by the system via 12v and then occasionally connect a USB for updates.
I also tried using 5v power directly to the 5v pin on the board but the UNO does not run the sketch that way.
Can I do this in a way that doesn't require me to disconnect the 12v power?
Using the barrel jack with >7V and USB should not be a problem. It switches of USB power that way.
Also
PickyBiker:
I also tried using 5v power directly to the 5v pin on the board but the UNO does not run the sketch that way.
That should just work a charm. So I think it's time to make a drawing (no Fritzing mess) of all connections and what else is connected etc. Because both powering the USB directly from 5V and connecting USB when powering it 12V to Vin / barrel jack does work. Only thing you should not do is power it with 5V (via the 5V / Vcc pin) and connect USB.
Do you have devices connected to the Arduino 5V? These may need a separate power supply when the Arduino is powered by 12V.
Found the problem.
I thought the solder points of the USB connector case were grounds, they are not. Moved all ground connections to the barrel grounds and all is working.
The image shows where I now have the 3 different ground wires attached (GNDs). It also shows were some of the ground wires were soldered when it didn’t work (Not GND).
Thanks for coming back to us 
The shield is grounded but via a small inductor. Because it's small that's probably what gave you trouble here
