how can I configure Core in order to use that external disk to save all the information?
Here, this data is stored in
~/.config/Bitcoin/Bitcoin-Qt.conf:
strDataDir=/home/simpleSliv3r/.bitcoin
So close Bitcoin Core, move that directory to a new location, edit this file, and restart Bitcoin Core.
Note that I don't have a GUI, I connect to my raspberry using ssh.
That's how I like it, most of the time a terminal is much easier for those tasks than a GUI. But if you don't know the basics of file operations (like
mv), you should probably learn that before using ssh.