Booting an old PC to SSD that the BIOS cannot see

[Robert Furber]

Instead of sending them to the landfill, old PCs can be upgraded 
dramatically by replacing or supplementing the HDD by a SSD ..specially, 
a NVMe SSD. In my case, the NVMe SSD is mounted on a PCIe adapter card.

The catch is the BIOS in older PCs cannot see or be aware of a NVMe SSD. 
They can only boot from a HDD. However, after booting, the NVMe SSD 
becomes visible and accessible.

The challenge is to figure out when, during the Grub boot, does the NVMe 
SSD become accessible, and then figure how to reboot on it. For 
instance, would it be possible to set up a customized Grub on the HDD 
that would chain a second Grub on the SSD ..as soon as the PC becomes 
aware of its existence? Sort of along the lines of "Chain-loading and 
OS". But, instead of chain-loading Windows10, it would chain-load 
Ubuntu20 on the SSD?

I am really green. Up until 2 days ago, I had no idea Grub even existed. 
Since then, I have been reading up on Grub and I feel overwhelmed, under 
the impression that it could take me months to learn enough about Grub 
to figure out how to boot from my new SSD.

Can anyone help me or point me in the right direction?

BurnsM