I have a machine I built, 2 or 3 years ago, around an ASUS P8Z77-V LK mother board. It's used for video editing. This Summer I went through a boot problem caused by a Microsoft Win 7 update, which was resolved by turning Secure Boot off in the UEFI. It works like new again, but I find that it will not search down through the boot sequence to find a good boot device (not sure it ever did). In past machines I always put the optical drive at the top of the boot sequence and my preferred OS in second place. That way the machines would boot from the optical device if a bootable disc were installed, or drop down to my preferred OS if no bootable disc were present in the optical drive. This UEFI will only boot from the device at the top of the boot sequence. If the first device isn't bootable, or has no disc, it throws an error message. The only way to boot then, is to boot into the UEFI and move the desired OS device to the top of the boot list.
I'm beginning to believe that this is just the way the UEFI is designed to work. Does anyone know more about the UEFI boot? Is there a setting I'm missing?