How do you redirect any steam game saves from the document folder to my external hd

If the game is written to save its files in only one place and look only there for them the next time you open it, there won't be anything you can do to change its behavior other than persuade its developer to rewrite it.

On second thought, this trick might be worth a try:

Copy the game's save folder from your Documents folder onto the external drive, then delete it from the Documents folder and empty the Trash. Then make an alias of the folder on the external drive and move the alias into your Documents folder on the internal. See whether that works.

How will partitioning your drive help?

I don't understand at all what partitioning will do for you, but I hope it does what you expect. Good luck!