Credits to Mark, and here is how I did it, this is my /etc/fstab:
proc /proc proc defaults 0 0
/dev/mmcblk0p6 /boot vfat defaults 0 2
/dev/mmcblk0p7 / ext4 defaults,noatime 0 1
/dev/sda1 /media/pi/USB ext4 defaults,rw 0 3
# a swapfile is not a swap partition, no line here
# use dphys-swapfile swap[on|off] for that
I've just added the line that starts with /dev/sda1. What I've read on forums it is important to keep in mind that you can apply the trick above only when you are using 1 USB storage device. When you have multiple, you have to work with UUID's. It's well explained here.
Oh and for an explanation of the different columns you see in my code you can check here.
