As far as a failed drive goes it is like this. You have one drive that is called a parity drive and is not used for data. I am not sure of the exact how it does it, but basically if you lose a drive and put a new one in, based on what is on the parity drive and on the remaining good drives it can tell what is missing and replace it.I ordered the WD drive.
Pardon me but I have never been able wrap my mind around just how a server will always have everything backed up in the event of a drive failure. I have always chosen the simplest solution. As it is now I will have one 6tb drive with everything on it and my 6 1tb drives as backup that for the most part would remain unplugged except when I copied the new backups on one of the 1tb drives.
Another reason I never set up a server, I thought it would be a waste of power having those drives spinning 24/7. I sometime won’t watch a movie for weeks and then I could spend a whole day watching stuff. It kinda sucked having everything spread out over 6 drives. Now everything will be on the one drive and at the rate I’m getting new stuff will last a least a couple of years.