Buy your NAS and RAID drives on different days...

[FlyingPenguin]

Or from different suppliers.

Steve Gibson passed on an interesting tip from a listener on last weeks podcast:

Since defective hard drives tend to come from the same run (if one HDD is out of tolerance, chances are ALL of them built that day on the same machinery are out of tolerance), it's best to buy your RAID drives from different runs. Buy them days or weeks apart, or from different suppliers to prevent having all your RAID drives die out around the same time.

[psypher]

That used to be a big issue years ago, but nowadays it's less of an issue. Most of the recent issues have been around firmware, which affected more than just a batch here and there. Nowadays, to have multiple drives fail at the same time is pretty rare and most likely due to other factors unrelated to the batch of drives.

They publish some interesting stats on drive failures.
https://www.backblaze.com/hard-drive.html

Love me HGST