Dual boot MB; 1 MBR, 1 GPT drives, GPT won't boot, same O/S

[videobruce]

Specs;
Gigabyte GA-970A-D3P Rev 2 older MB (2016) AMD based system.

Main drive; 250GB SSD W7 Pro x64 MBR no hidden partitions

Backup; brand new 4TB same O/S GPT one created System partition using Disk Part .

Backup is seen in the Bios, but won't boot even if the SSD is disconnected. The MB is advertised as "dual boot". No custom settings, no overclocking etc. I've tried different Boot settings, the few that are there and no difference. The system always boot to the MBR. I haven't reset the Bios since I don't do custom settings, but I considered it.

The original system was the SSD and 2TB MBR Backup drive (no hidden partitions), same O/S's and that worked for eight years. Both drives have other storage partitions.

Any other details, please ask. The basic difference is one MBR and one GPT drive.

[AGTDenton]

I should point out I haven't done dual booting since Windows 2000/XP days with boot.ini. Once I found virtual machines I never looked back.

When you say the MB supports Dual Boot do you mean Dual BIOS? Dual Boot isn't a motherboard feature.

In either case, I'm amazed this tool is still going but EasyBCD may be your friend. https://neosmart.net/EasyBCD/
You have to inform the Boot Manager there are two OSs to boot from, or repair any BCD corruptions.

I'm not sure if you're going to have to configure the BIOS to enable both UEFI and LEGACY. But I would try the EasyBCD route first.

Before you do this though, take a system backup because BCD can break things if used incorrectly.