Also why it's a good idea to not buy all of your HDDs for your NAS all at the same time. Or at least not all from the same batch.
Nothing worse than your NAS and all of the parity drives going down at the same time!
Also because of this, running a NAS is so expensive. I think I have about $2000 worth of drives in my NAS, which equals out to be $400 per year in replacement drives.