I really hope you see this because I had the exact same issue as you literally last night, I was up until 4am on the line with EA help, but in the end I did it by myself!!
---- IF ANYONE DOES THIS PLEASE MAKE SURE YOU BACK IT UP AS I DIDNT AND LOST ALL MY SAVES!! -----
1. Unplug your external hard drive, completely.
2. On the EA app, download the Sims 4 again (as it should say its uninstalled now)
3. I can't remember whether it tells you this before or after you install it, but make sure that the install location is in documents.
3. Open the Sims 4 (really hoping that worked for you)
4. Go onto Game Options and make sure mods are enabled.
5. Close the Sims 4, plug in your external hard drive, and open "Terminal" on your Mac.
6. Type in this exactly, but change "USB" to the name of your hard drive (no spaces!)
mv ~/Documents/Electronic\ Arts /Volumes/USB/
7. press enter, and do the same thing for this command.
ln -s /Volumes/USB/Electronic\ Arts ~/Documents/
8. Now if you check your documents folder, it should have a little black arrow on Electronic Arts, and in your external hard drive, you should have the Electronic Arts folder.
9. Play your game!!
If you've already fixed it, that's great, I really hope this helps anyone cause I swear I was up for hours just for it to be an easy fix lol
I'm aware of this, however, before I did this, my settings were correct, this was taken before I fixed the issue with that method, I'm no expert, for definite, but this is what worked for me and if its gonna work for other people, so be it. Just trying to help lol..
