I have a Dell 7920 tower workstation with the following disks and operating systems 

* 256 GB NVMe - Windows 10

* 512 GB NVMe - CentOS (7.6 I believe )

* 1 TB SATA SSD - Ubuntu 20.04)

* 4 TB SATA hard drive - CentOS  (7.7 I believe)

I was never happy about the way to select the OS I wanted to boot, as I never see all options presented. Instead I had to disable a disk in the BIOS, which then obviously prevented that OS from booting.

I had to change the motherboard. Since then, despite the BIOS detects the 1 TB SSD with Ubuntu on it, there's no way I can find of booting Ubuntu.

I put a Ubuntu USB stick in the workstation, booted from that. It detected Ubuntu 20.04 is already installed.

So I know Ubuntu is on the disk, but I'm at a loss as to how to get the 7920 to boot it. The operating system never shows up in the BIOS as one I can select. I know its possible to add a bootable drive in the BIOS settings, but one needs to know exactly what to put, and I don't have a clue.

Any suggestions how to get out of this?