The fstab tells the system what drives to mount and to what mount point during boot. The last line was telling the system to mount a /dev/sdb1 to /media/dol/Gwenou. With one or the other missing or both, the system couldn't complete the task and hang. Putting a # at the beginning of the faulty line comments out the line and makes it not function. Commenting out a line in a configuration file is safer to do, in case of undesired results is easier to go back and change vs deleting the line.

Ok, understood.
I remember researching ways to automatically mount one of my HDDs on my computer, because it wouldn't without me telling it to. But i don't think i actually tried to do anything to do it... and certainly not on my laptop.
As you can guess /media/dol/Gwenou was a mount-point for a USB stick, but i don't remember creating it. It was a windows formated usb stick i tried to reformat in exFat first, and in ext4 later, but which still wouldn't work. Something might have gone wrong during that time.
Any guess at what caused this drive to mount onto the wrong mont point ?

I'll return here to tell you if i run into any new problem with my computer

I don't have the first clue what happened. Probably has something to with the copying of files back and forth.

Various methods of removing old kernels is explained in the Ubuntu Handbook at the site below.

http://ubuntuhandbook.org/index.php/...-ubuntu-16-04/

Ok so i looked a little bit into it, and at first it seemed my computer actually suffered from the same problem than my laptop. But it seems it doesn't...
Because of that, i started a new thread here and will mark this one as revolved.

Thank you very luch, you two, for your time and your support