[Not sure this is what you're looking for but....]

This is what I've done (ignore the attract mode bit) -

Runs it all from the HDD (SD card is still needed), has been working fine for me.

@wilhood
This is the problem I want to fix on my raspberry as my external hard drive is better then my sd card. Have you gotten it to work?

Thank you for the help!

@brookes

https://retropie.org.uk/forum/topic/3/read-this-first

@herb_fargus It's not a 3rd party image, it's someone's preconfigured noobs setup with 3 partitions: one with Retropie, one with Kodi, and one with Raspbian. It's still the default Retropie image on top of a headless raspbian os. It was done this way so you would have less overhead when running Retropie (rather than having Raspbian with the GUI with both Kodi and Retropie installed and running on it).

i would use our guide instead (which is pretty similar, but has some crucial extras) - https://github.com/RetroPie/RetroPie-Setup/wiki/Running-ROMs-from-a-USB-drive