For a number of years, Macintosh computers have been able to calibrate two displays. By and large this is not the case with PCs. Most PCs have a single channel video controller. These controllers have a single look up table (LUT). In the PC world it is a general requirement that each display have its own video card if dual monitor calibration is to function properly. That said, you can use your external monitor as your calibrated device. Simply drag the application onto the the external monitor and affix the measurement device. The software will automatically produce the calibration and or profiling targets and create a profile for that device. If you wish to re-assign the internal calibration you can do this from the desktop. Right-click / Properties / Settings / Advanced / Color Management. Windows does NOT reload profiles after the system has started. It is necessary to restart the system after changing profile defaults.