Ok, got it.
After all this stuff, where will these files be? Will they already be located in sdb8 and my computer will already boot there, IF everything worked out as expected? Or is this the first step to accomplish this goal, next to be done tomorrow?
Under the path /home/, like before, except this time /home/ is not just a normal folder like in your original setup, but it’s a mount point for the ext4 filesystem on /dev/sdb8.
If everything is done, and your fstab is in order (correct format, no duplicate or redundant entries), then a reboot should work like normal.
If and only if you’re 1,000,000% sure you’ve successfully migrated your home contents, then you can safely remove everything under /mnt/ (the old stuff).
IF AND ONLY IF this was a success, then you MUST (this is not optional) unplug your headphones, crank up your speaker volume to the highest level, and blast this video on repeat.
This is NOT a suggestion. It’s a hard requirement.
Ok! I’ll try to strictly obey the rules!
In the meantime, thank you all guys for your help and patience and go get some rest! (Or coffee…)
Soooo, everything is working here!
I’ve checked fstab, run “dry run” and reboot. It went straightforward to log in and then users. Especially my user runs now a bit slower, but likely due to the fact that now /home is in a HDD, and not a SSD as before. Apart from that, all is running great.
Now it’s time to delete the 200 GB in the /mnt/ folder. Should I simply use the rm command? Should I delete only the folders inside it or /mnt/ as whole? Any special hint? I can also do the same to /migrate/, right?
Thank you all again, it was quite a journey!
And, as requested:
I tried it hard! But it keeps saying “unavailable video”…
THAT IS UNACCEPTABLE AND A TRAVESTY OF ALL THAT IS GOOD AND JUST IN THIS WORLD. 
You don’t want to remove the actual folder /mnt, since it’s part of the standard filesystem tree, and is very useful.
To play it very safe, just double check inside with ls -la /mnt/ to make sure nothing else exists besides the original home/user folders. If that is all that is inside, then you can safely remove the contents with:
sudo rm -rf /mnt/*
THE ABOVE COMMAND IS DANGEROUS AND DESTRUCTIVE. DO NOT RUN IT IF YOU CANNOT SEE THE ENTIRE COMMAND (such as viewing it on your phone.) EVERY CHARACTER IS IMPORTANT, EVEN THE VERY LAST ASTERISK. IT CANNOT BE UNDONE AFTER YOU RUN IT.
THERE SHOULD ONLY BE THREE SPACES IN THAT ENTIRE COMMAND, AND EXACTLY WHERE YOU SEE THEM.
sudo ← run it with root privileges-r ← recursive removal (delete everything within the target)-f ← force removal/mnt/* ← wildcard to delete everything inside of /mnt/, but not /mnt itselfMake sure migrate is truly an empty directory at this point.
ls -la /migrate/
Make sure it’s not somehow being used in an active mount.
mount | grep migrate
Then use the rmdir command, which is much safer than rm -rf, in that it will only delete a directory if it’s empty.
sudo rmdir /migrate
I found an alternative solution to the most pressing issue at hand.
All good! Checked everything, these were the only files and folders indeed, so I entered the commands and deleted everything.
So, thank you again! Linux communities are truly amazing!
If you have any other link???
If the second link doesn’t work, then we need to declare an official State of Emergency. 
Sorry! Now I noticed the second link and… it works! Mission accomplished 
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