I intend to check out the Youtube and see if I can do better, but on my Lenovo found a simple fix for a problem which had many times before (install a distro, doesn't show up when rebooting): Hit the F12 key and the boot disk order comes up. In my case I have my main daily driver (Manjaro Gnome) on a 500GB SSD (sdc3), and have installed Solus on a 250GB extra SSD (sda). When I hit F12 it shows a list of HDDs albeit it uses the manufactures nomenclature not the sdxx system. If I want to boot into Solus I have to specify the 250GB SSD because BIOS I have the 500 GB coming up first in the Boot Priority order.
Am not advanced user so have not yet managed to understand how to switch over to EFI when first installed using legacy without having to re-install. I am hoping the Youtube will get me into universal EFI so that I don't keep having to hit the F12 key. Meanwhile, it's a simple solution which adds only about 5-10 seconds to the booting up process.