Apologise for bumping, but 1) perhaps I got few more clues and 2) I would LOVE if anyone might have suggestions for fixing.
1)
It seems I cannot even delete files from my two NTFS drives despite acting as administrator. Even if using terminal with for instance rm statistics.xml I get rm: cannot remove 'statistics.xml': Read-only file system. I somehow expect it is the same reason I cannot run Steam games, although I DO NOT UNDERSTAND why a game installed on a ext4 drive will not launch.
This brings to memory that when I first connected the old HDD I think my PC tried to boot Windows from it. In any case, after adding it suddenly it took forever to boot (with a black screen), and eventuelly I cut the power, went to BIOS and edited boot priority. Since then I haven’t been able to run Steam game as mentioned.
I found this and this regarding read-only NTFS drives that suggested to fully boot Windows and fully shut it down. I did, and to my horror I no longer can boot EOS. I highly think Windows - some frigging how - overwrote my /efi partition despite EOS being on a completely different hard drive? How can that happen?
In UEFI/BIOS, my EOS-SSD is no longer called “Linux Boot Manager” (GRUB, right?) but Windows Boot Manager. If I boot from it it will simply not boot.
2)
I found a few threads to follow to see if I can recover it.
BUT: can someone explain what on earth happened? And do you think my theory is valid?
I genuinely thought that dual booting (which I understood was not recommended) referred to having two OS’es on same physical hard drive, thus I kept the OS separated.
Again, thanks in advance. Very much appreciated for any help.