@chrdaman21 Do you want to move the Sims 4 program files to your external drive, or your user data (saves, custom content, etc.), or both? For the program files, you'd need to uninstall the game and reinstall it onto the external drive. This shouldn't delete your user data, but it's still a good idea to make a backup copy of the Sims 4 folder in Documents > Electronic Arts first. You can just copy the entire folder to the external; put it inside a subfolder to avoid confusion.
If you want to move the user data to the external and have the game read it properly, you'll need to use a symbolic link:
https://crinrict.com/blog/2020/02/moving-windows-documents-folder-to-external-drive-via-symbolic-link.html
If you get an error with the command offered, write out the full file path to the user folder instead of %userprofile% . You can see the file path by right-clicking the folder and selecting Properties > General, then looking for the Location.
If you're moving both the program files and the user folder to the external drive, it's critical to use subfolders so that the files don't all end up in the same place. For example, you could install Sims 4 into D:\Games and direct the user data to D:\Data\The Sims 4.
If you have any trouble with this, just let me know what you're trying to do and what isn't working.