Since you're using an external drive, in your BIOS try to search for an option that lets you boot from USB devices. If you can find it, disable it. What I think happens now is that your BIOS is set to boot from USB devices and when it recognizes a disc in your PX-716UF it tries to boot from it, which fails and hangs your computer. By disabling this option your problem will hopefully be solved.
Let us know if it works!