I'm currently running two 1TB hard drives, one of drives for the operating system and other main files and the other hard drive all the game installations. I'm also running two 2TB hard drives and two 3TB hard drives for storage purposes. I'm currently running this setup because it was cost effective and I bought all these hard drives when they were on sale and it currently gives me adequate amount of storage for my media.
These 4TB Western Digital Red drives are interesting to me because it would allow me to consolidate my drives and I would eventually be able to get a SSD drive to replace my current drive to run my operating system and gain a great boost in terms of responsiveness.