fstab is a FILE located under /etc
to edit nano fstab while in /etc dir.
to view more fstab
LABEL="usbdisk1" /home/pi/RetroPie ext4 nofail,defaults 0 0
As long as an external drive connected is labelled usbdisk1 it will get mounted to home/pi/RetroPie.
We can then cd into home/pi/RetroPie.
This way we can boot pi with different drives.