clean up COM ports in use

Run Device Manager from elevated command line:
> set DEVMGR_SHOW_NONPRESENT_DEVICES=1
> devmgmt.msc
Enable “Show hidden devices” in the menu, and uninstall grayed-out COM ports.

출처 : http://superuser.com/questions/408976/how-do-i-clean-up-com-ports-in-use

귀찮으니 시스템 환경변수에 전역으로 DEVMGR_SHOW_NONPRESENT_DEVICES 값을 등록해 버림~