Ok, thank you very much Gregory.
So:
I’ve first tried out sudo systemctl daemon-reload , which doesn’t seem to have done anything (even after reboot, the 10tb changed from what was sdc before to sda now even before using the command). I do not know what the computer was trying to do, but it seems that it was trying to address 10tb both with its old and new sdX address…
So, I’ve created a new mount point and addressed it as 10tb’s new Mount. After a system reboot it now just mounts.
.
I would have tried it on my own, but it’s good to first ask with more knowledgeable people.
How did the say go? “If you ask you are stupid for a minute, if you don’t ask you are stupid forever”?
Better to ask than to nuke my HDD.
.
.
.
As a closure, speaking about nuking data:
Sata (Data) drives are hotswappable, meaning that they can be plugged in and out dynamically, but when I did so with my Windows 10 Backupping Computer I first had the 2tb drive on “the Sata Data cable” (copying off its files in the backup) and then plugged in, on the same Sata Data, the 4tb one.
It seems that Windows 10 got confused, corrupted the 4tb’s data and tried to write upon it 2tb’s folders.
It’s all good, I already had MANY separate backups of all that was important (only lost some games from Steam, no biggie 
) but it was still a bother as HDD average speeds are 120-140Mb/s.
Wanted to share it here, because it should be shared somewhere, and this seems like a pertinent place (now 4tb is an Ext4 drive, mounting on boot in Fedora KDE).