This post is a bit older but maybe someone is still interested.
I asked myself the same question: What to do with 4 unused sata drives (and a raspberry pi (2+))?
So i decided to build a NAS Server for file sharing, TimeMachine Backups and stuff like that.
To power the four 2,5" hard drives and the pi itself i bought a 5-port USB powerport.
And with a sata to
I build a rack to mount the drives with wood laying around. Meanwhile I mounted the powerport and i want to paint the rack.
Its not fast, but for me this is totally enough. soon i will upgrade to the next raspberry pi 4 so that the speeds will rise