Ok are a few things here that makes the difference.
If you are on Windows I suggest you edit your file in your raspberry pi as you cannot see your file properly.
To edit your file under Rpi (linux) environment you can use this command:
sudo nano /etc/emulationstation/es_systems.cfg
Please note you must exit Retropie first in order to get your command prompt.
If you use linux in your desktop (Ubuntu ou other operating system) you can edit your file with other application like gedit but I’m affraid it’s not your case.
Because NANO has a very simple interface this link will help you in your find and replace journey:
You will find lines as it follows
<path>~/Retropie/roms/EMULATORNAME</path> (something like this, I don’t have it in my mind right now as I’m outside my house)
And what you need to do is to replace this address to Your USB PEN Roms Directory.