You might want to look into Unraid from here Unraid

They have 3 versions Basic (3 drives), Plus (6 drives), Pro (16 drives). With the basic version you can set up 3 drives but one being used as parity. So if any single one of your data disks fail, it can be rebuilt without loss of data. I believe you can start with 2 drives, then when the one drive is full, just pop in another. I use it for my media server which has a total of 7 x 500GB drives right now.