If you are looking to build a NAS system, have you considered what level RAID you are going to employ? With 2 matched drives you can go with RAID 1 which gives you the space of 1 drive but will keep working if one drive fails until the failed drive is replaced (some models even allow hot replacement). If you can go to multiple drives, then you could have a RAID 5 system whereby if any one drive fails then the system will still work - or even RAID 6 which will still work if 2 drives fail. See https://en.wikipedia.org/wiki/Standard_RAID_levels