It would definitely be worth posting that over in the EA App forum in the hope that someone relevant might see it - EA app Technical Issues & Bugs | EA Forums
What format is the drive? Did you reformat it before using it for Sims 4 stuff?
OK, I'll post it there. Thans again so much for all your time and effort!
To answer your last question: The format of the drive is ExFAT.
It hasn't been reformatted since it's brand new. I've already put some files on it (regular files like invoices and such) for work purposes.
There are no games installed on it.
Hmm, I wonder if ExFAT is the problem? It's a format that both Windows and macOS can read but APFS is the Apple format that is preferable. Brand often bundle their own software/backup/other stuff onto the drive so by reformatting it straight out of the box you're getting rid of all that crap and starting fresh. ExFAT is not compatible with Mods when users store their Electronic Arts folder on the external and create a symlink in their Documents folder. That would be a really interesting thing to test and could also be fed back to the EA App team if it fixes the issue - back up your important stuff on the drive onto another one or on your internal drive then reformat the drive to APFS using Disk Utility
