I guess I don't understand what I'm doing wrong. I format the card in FAT and burn the image from the Dropbox link with Etcher. There are literally no other extra steps in the process that I've taken. It loads the UI on the Retron and plays games from cartridges. The only thing that is missing, as far as I can tell, are the roms, which are definitely in the Games folder since I can see them on the computer - they just seem invisible from within the Retron.
What format roms are you using? Zipped? Unzipped?
You are burning the image right, sounds like if it boots correctly. Try downloading the harmony cart ROM pack in ROM's collection over on Atarimania, and putting them in the correct folder.