@themazingnesshttps://retropie.org.uk/docs/Debian/ (at least that one worked for me on 20.04.1 LTS a few weeks go), though co do [final edit:] to build some additional (optional/experimental (can't remember which one, could have been lr-mame 2016)) packages, it was neccessary to install python 2 (which is EOL, and therefore not included with UBU20).
Clicking on the 3 vertical dots at the bottom right of your post should allow you to edit the post.
Regarding your issue, it looks like the disc is not fully mounted when EmulationStation starts and the folders are not available. Depending on how large the disc is, this can take a few seconds. Edit the autostart script in /opt/retropie/configs/all and insert a delay before EmulationStation starts:
You can adjust the delay (in seconds), depending on how much time it's needed before EmulationStation starts.
Thank you for responding so quickly to my question(s). The menu with the dots only gave me the option to delete the post before when I checked it but now gives me edit options. I'm not sure why, hopefully that was just a glitch.
As for the ES start delay idea, I'm about to call it a night but will try this tomorrow and let you know if it takes care of the problem.
@mitu Just did final test. It all worked out perfectly. Cache has been moved to the external drive using the path to retropie-mount. The only thing that was weird was after nano-ing into the config.ini, I got the same error message, even after the MicroSD was cleared of the cache. It was just a black screen with the error message at the bottom. However, after hitting ctrl + r to give a command to read a file, I just typed in config.ini into the prompt, and then the file decided to open and let me edit it. Aside from small bump, everything worked out perfectly.