I would think that the BIOS would just skip t the next drive when the first did not boot. I'd check the BIOS settings, though. Whether it will "function OK as the new main" is a different story though. It will depend on the software running and the paths described to the OS from the hardware. Best way to tell is to try it before you need it (unplug the first one and attempt to boot up).
OK, cool. The DVD drive puked and I got to wondering if it was possible to put in a 2nd drive as there’s no provision for a 2nd drive physically compared to a desktop’s configuration, which led to my questions. It’s good to see that it’s possible. Thanks for your time!