when you are in the setup menu (where you choose to set HDD as the priority for boot) have you ever tried clicking on the HDD in order to force it too boot the OS (just as a one time thing)? I literally mean just double click on the HDD (at least on ASUS motherboards that's how you do it) which should then allow you to boot from whatever you double clicked on (may be different for other motherboards, play around with it but force it to boot from the HDD without restarting). If you could do this and let me know it would help in diagnosing the issue. If when you force it to boot from the HDD the OS boots just fine then there is something wrong either with the boot priority or the motherboard itself but if it doesn't boot from the HDD when you force it then the problem either lies with the install of the OS on the drive or with the drive itself.