In order to boot from your external HDD, you first need to select this from BIOS. To enter the Boot menu from BIOS, you need to restart your PC and press the Boot menu key after the memory checking process. The Boot menu key differs from one computer to another depending on the BIOS version. Usually, it's one of the following keys : F1, F2, F10, DEL. After you accessed the menu, select your external HDD for the Primary boot device. By doing this, your computer should boot from the external HDD and start PCCloneEx utility.