Had I been using RAID5, I would've lost all 18 terabytes of data. But since I was using RAID6 which has TWO redundant drives, everything was safe. The NAS was able to both rebuild the first drive AND remap the bad sectors of the second drive.
I point this out because RAID5 and RAID1 are still by far the most popular RAID configurations, and they are a Really Bad Idea. Error rates per terabyte remain the same, but we can fit more terabytes per drive every year. That means error rate PER DRIVE increase every year. Capacities are so high now that it becomes probable that one drive will experience an error while rebuilding a RAID array due to the failure of another drive. That means you must have at least 2 disks of redundancy for a RAID array, meaning RAID6.
In other words, RAID5 isn't sufficient redundancy anymore, you need RAID6.
Oh, an by the way, not only is RAID5 long dead, RAID6 will be dead in another 5 years as drives reach the point where THREE redundant drives will be necessary.
@erratarob welcome to 2009 queue.acm.org/detail.cfm?id=…
— Ignacio Marambio (@imarambiocatan) October 2, 2012