Fujoshi_Love23 You can't simply cut and paste the game's program files to your external drive—the EA App won't know where the game is installed anymore. If your internet is slow, you can move the files and tell the App to "download" to that location, and it may simply verify the files and finish the install process rather than redownloading all that data. But the more reliable approach is to uninstall the game and reinstall it to your external drive.
If the problem is your user data (saves, mods, etc.) rather than the program files, have you created a symbolic link? You'd need to do so to tell Sims 4 where to find that data.
