I have two Arduino Uno's. One of them works fine, and shows up as "com3" under Serial Ports. The other, however, tries to work as "com6", but I keep getting the error "Serial port com6 not found." I have tried restarting the program with the Arduino plugged in (which was suggested for similar problems), restarting the computer, and hitting the Reset button on the Arduino. Also, the Arduino that works shows up under the Device Manager, and the other one doesn't.
If it doesn't show up in Device Manager, how do you know its getting assigned COM6?
COM6 is what the Arduino software tries to connect to when the 'faulty' uno is plugged in.
The Arduino software has no idea what which COM ports does what. So something else might be creating "COM6" and the IDE picks it up as the only serial port.
You need to solve the problem of why your board isn't showing up in Device Manager.
I forgot to mention that the Arduino gets power from the computer just fine. I've also tried it with two different computers and had the same problem.