ok, good news!!
i had a play around, and i can tell you the guide work!
but your drive must be formatted to ext4.
this can be done by flashing an sd card with Raspbian wheezy or jessie.
when its booted, open the terminal and you’ll want to download and install parted
sudo apt-get install gparted
once all is done, it’ll be somewhere in the menu, open and log into it password is raspberry
and then format the usb drive that will need to be selected from the dropdown menu on the right of the app window,
then follow the guide, use
to find your UUID & the drive number “sd??” (probably sda1)
then when you get to
sudo nano /etc/fstab
replace your new UUID
it seams a lot of things to do, but by using the UUID, you can safely add other USB drives, without them getting swapped around
after that you can use this es_settings.cfg I’ve zipped below
next you’ll need to get write privileges
sudo chown -R pi:pi /media/USBdrive
and then create a folder called roms and in here add your rom folders