Useful information lacking, useless information present. We don’t have your partitions, filesystems or installed operating systems memorized, and can’t see over your shoulder. What does “15.4 is still a grub boot menu system” supposed to imply? What partitioning is present on all the installed drives. How many ESPs are present? How are you booting now? Where is Grub now? What does sudo efibootmgr report?
It may be nothing is necessary but to make an appropriate menu selection after striking your PC’s BBS key during POST, but you might need to enter BIOS setup to wake the BIOS up to the presence of added SSDs, and/or move one to top of boot priority list.
Like 15.4, 15.5 will be a minor release. How booting works won’t materially change, if it changes at all.
MrMazda, these hard slaps on the back of my head are getting old!
I’ll NEVER be able to suit your posting needs! Why even bother with another head slap?