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.

- - - Updated - - -

Here is the Bios boot mode screen.

- - - Updated - - -

I did reset the Bios to 'optimized defaults", but no difference.
Another 'oddity', the SSD drive that does boot, doesn't show on that page I posted even thou it shows in the boot menu and under System Info.

 

[samuria]

Its not duel boot its is duel BIOS. Its not clear what your trying to boot win7 win10 other if one is uefi and the other legacy one one boot as bios will be wrong what systems are you trying to boot and what are they set to?

 

[videobruce]

Ok, typo. It's W7 Pro x64 on (originally) both MBR bootable drives. The new drive (4tb) forces GPT /UEFI.
Apparently, the MB didn't like two different drive types.

Long story short, after the new drive was switched to GPT the same was done with the former MBR drive. Now, all is fine I can boot from either drive like before.

 

[MisterEd]

Ok, typo. It's W7 Pro x64 on (originally) both MBR bootable drives. The new drive (4tb) forces GPT /UEFI.
Apparently, the MB didn't like two different drive types.

Long story short, after the new drive was switched to GPT the same was done with the former MBR drive. Now, all is fine I can boot from either drive like before.

I have a computer very similar to yours. The 3 internal drives are MBR while the external one is GPT. This computer started out as Windows 7 only which explains why it is MBR. The external one's size is greater than 2TB so GPT is required. My motherboard doesn't even support UEFI but a modified EFI which has a BIOS option to boot from GPT drives larger than 2.2TB. I never booted from a drive larger than 1TB so never tried that option.

I have a number of computers. None of them have HDDs or SSDs larger than 2TB. The main reason is making backups. Backing up drives larger than that make for very large and expensive backup drives. Also, I like to try to keep Windows and user data on separate drives.

Internal
Disk 0: 1TB SSD MBR (Windows 10 Pro)
Disk 1: 1TB SSD MBR (Data)
Disk 2: 1TB SSD MBR (Windows 7 Home)
External
Disk 3: 5TB HDD GPT (Backup)