Het is opgelost.
Door deze stappen te volgen:
In Origin > Installs & Saves > Game Library Location click on Change Folder
In the left side click on Computer then double click on the ‘/’ that appears next to a picture of your hard drive in the right side. You should see four folders in the right now - Applications, Library, System, Users.
In the field at the bottom, where it says Directory, type in /Volumes/[the name of your external] then hit Enter. For example, my external is called Backup and shows Backup when plugged in in Finder, so the file path is /Volumes/Backup.
The window will disappear and reappear again but you may not see your external. That’s ok, quit Origin then reopen it. You should get a Gatekeeper message popping up asking for Origin to have permission to access files on a removable volume. Just click Ok.
Now repeat steps 1 to 3 and you should see the contents of your external in the window.