Thanks for your answers jailbait, I think I've found it:
By default, an ext4 filesystem is created with the lazy_itable_init option enabled (more info in the man page). What does this mean?.
The inode table is not initialized when the filesystem is created (which speeds up filesystem creation), but the kernel will have to initialize the filesystem in the background when it will be first mounted.
I guess that while the bigger the filesystem, this reading-writing behavior will last for longer, and if it is in a backup USB drive like mine, it will be more noticeable than my system's HD current read-write operations.
As a reference, in a 855 GiB ext4 filesystem it stopped in less than 2 hours.
I still wonder what happens if you umount the filesystem before it ends up initializing the inode table for the first time.
I hope I've been helpful to any readers, if so, add to my reputation
I'll mark this thread as solved as soon as I find a way to edit the tittle, I promise.