I have decided to reinstall my Lubuntu OS and to split on the occasion my partition so as to have a secondary one where long-time files would be stored. I guess what I wanted followed a logic that was close to my experience as ex-windows user. I just wanted a new non-system partition visible in the left side pane of the file browser.

When trying to install the system onto the smaller partition for which i set / as mount point, I was prompted to set a different mount point for the other: i had already used / for the one that was not meant to be the system partition because i didn't new what / meant. Not knowing what to do I selected /boot for the second and went on installing on the smaller one with mount point /.

All was ok except that now the larger/secondary (/boot mount point) partition is not visible as I expected it to be.

But this doesn't mean that it is completely hidden or unaccessible. I found that /boot is in fact the location for this partition: when i go there i see i'm within the 30GB/second partition. creating a folder there and adding that folder to side pane (create shortcut) in Thunar, makes in a way that partition immediately visible and accessible in file manager's side pane the way i wanted to.

but this is still not a solution, for i cannot access it as a partition (unmount it, etc)

In Gparted it is: