Hi all. Just adding to this thread to mention that I was also having an issue where, when attempting to manually append a disc in Retroarch, no ROMs were appearing in my PSX ROMs folder. The above solution worked for me. This is the step-by-step for the technologically challenged (like me):
Using your computer, navigate to the "configs" folder in your RetroPie drive
Go to the folder of the emulator for which your having an issue (in my case, "psx")
Edit the retroarch.cfg file by right clicking it and choosing to open it in some kind of text editing program
Add this line to that text file, substituting the 'psx' part for the relevant system in your case:
rgui_browser_directory = "/home/pi/RetroPie/roms/psx"
Save the file
Restart your Rasberry Pi
When you go to append a disc, you should now see all of your ROM files.