I think storing cc on an external drive as a
backup is always an excellent idea, but I am not sure how you would do it for your game, especially if you use script mods. I don't know how your computer is configured, but my gaming computers always have a small c: drive so I usually install programs to the d: drive and just use the c: drive for my sims games, keeping an eye on the GB left.
Just try it and see.
backup your game as well to an external drive (tray, saves, screenshots, custom music, mods) and then see. Can't hurt.