It varies from brand to brand, which ssd and hard drives are you looking at ?
When you have steam the games and updates it comes with can be pretty big.
My suggestion is you keep steam seperate from the ssd drive and let the os have exclusive rights to it.
Meanwhile let the other hard drive keep all of your games and main apps to load up from.
Unless you don't intend on moving the cords for the external drive or having it drop by accident, I wouldn't recommend that.