[lug] How to boot from another HD?

[Michael Hirsch]

My desktop computer died, so I bought a Dell 3880.  Installed ubuntu
(kubuntu, actually) 20.04.1 on the SSD.  Then I brought my 1TB drive
over from the dead computer and mounted it at /home and all my files
are back.  Yay!

Actually, a bunch of stuff isn't working as expected, so I thought I'd
try running my old OS.  I put the SSD from my dead system in my new
computer.

I can see the drive, but for the life of me I can't figure out how to
set things up to boot from it.

I used to be a master of editing lilo or grub files, but it has
changed in the last 10 or more years, not to mention "secure boot"
which scares me, but seems to work.

Can someone point me to a tutorial, or talk me through booting to this
alternate hard drive?

Thanks,

Michael