New Linux install; other Linux drive no longer responsive

(See Edit 2 for another possible cause for this problem.)

A few days ago, I installed Linux Mint on an SSD. This appears to have had very adverse effects on my main Linux drive, just recently updated to Kubuntu 26.04 LTS.

Kubuntu is now extremely inconsistent at startup, sometimes totally unresponsive to mouse and keyboard input, and displaying the desktop void of any icons; sometimes it doesn't appear as a bootable option in the BIOS or GRUB menu at all. Startup apps seem to launch normally, so I'm not sure if the system is flat-out frozen or not.

And at other times, I'm able to boot into it and run it without issue, as I'm doing right now.

Before turning my machine on, I often turn off the power supply and repeatedly press the computer's Power button to avoid unwanted motherboard caching, or unplug the Mint drive. Results vary.

When I installed Mint on the other SSD, I noticed it was altering not only its own drive, but (I believe) the Kubuntu one as well. From my reading, this is supposedly the new install interacting with the previous one's GRUB files. Apparently the only way to avoid this is to physically disconnect the older drive, which I did not do.

I have tried running fsck from another Linux drive to repair Kubuntu; despite notifications that errors were found and the OS was "modified", there is no perceptible difference.

I also have a Kubuntu live USB thumbdrive at the ready, but don't see any option to repair the install.

How can the Kubuntu drive (and ideally the GRUB menu) be restored to functional behaviour?

Edit 1: to my aggravation, the boot corruption appears to have stopped inexplicably, after simply repeating a number of manipulations. These may have included the commands sudo apt update and sudo update-grub, executed on various different Linux drives. Certainly, the recovery options (fsck, etc.) across these different installs were run multiple times.

GRUB's tendency to frequently, but not always, rename the Kubuntu drive as just 'Ubuntu' made matters worse since I also have a vanilla Ubuntu drive.

So I imagine the recovery tools eventually had some effect on the troublesome drive itself, but the BIOS's boot menu problem remains: the Kubuntu drive never seems to be available for booting, even though the motherboard still recognises it (its name is visible in the Storage Configuration section of the BIOS).

Edit 2: the boot corruption reappeared after a system crash, related to a graphics card I've been having issues with. This may well be the real cause of the booting problems. (The BIOS / GRUB question remains to be answered.)

It appears to have been resolved by booting into a previous kernel version and reinstalling the generic kernel with the following command:

Edit 3: No solution seems to permanently fix the problem.

In the end, I abandoned Mint and tried KDE Neon, which... installed itself not only on the drive I selected, but apparently also on my Kubuntu drive, wiping the original OS.

Discovering, around the same time, that Neon may soon be discontinued due to the departure of some key developers, I retreated back to my old Manjaro system and undertook some long-overdue updates, which of course led to more problems.

I've spent the last few days gradually climbing out of the crater and finding my way back to a functional setup.