Yes you can install steam games on an external drive or seperate partition, but it still requires you to sign into account to access them, and if you try to say play the same game on two different computers at the same time with the same account steam will force you to close one of them. I recommend buying games off GOG when you can since they are truly DRM free you may not get cloud saves or workshop content but you aren’t being bossed around by steam either.