I'm currently running three drives: One is my operating system and DAW program, one is my sample library (I just upgraded to Komplete 9 Ultimate, so this drive is now pretty much used for that only and I need to get one more for BFD2 and a couple of other libraries that won't fit anymore), and the third drive is a dedicated project drive, which is what I record everything to.
This is the best way to run things, as having one drive try to do too much can bog everything down in a real hurry. I can run sixty+ tracks of software instruments with my system just barely starting to break a sweat.