Dr_Quark:
I disconnected the Nano and I still get the serial port busy error message. Evidently the PC is maintaining COM36 as busy, even with nothing connected. I closed all open instances of the IDE, then reopened this particular app, but I still get the com port busy error.Anyone know how to kill this connection in the PC so I can reestablish it? I thought unplugging the USB connection was a fail-safe way to do this.
Any bluetooth stuff on your PC? It definitely is a PC side problem where something has grabed that com port number and the OS won't let any other application (like the arduino IDE) from gaining permission to use it. I assume you have tried restarting your PC?