Failures of various RAID modes.

"Should be dead" are charged words. It's probably more constructive to talk about our actual experiences with RAID and perhaps other NAS failures. I'll add my experience to dbott's.

I'm a home NAS user, and I've had a ReadyNAS since 2010 (and have had more than one for most of that time). At the moment I have 5 (which does seem a bit silly).

Altogether I've had 9 raid-years total with RAID-5 - 5 years with a 4x2TB in a NV+ and four more with 6x3TB in a pro-6. The NV+ is still using its original disks (after 5 years, with the head parking thresholds not tweaked). The pro array was expanded from 4x1.5TB, and has gone through at least 6 disk replacements (as the entire array was upgraded to WD30EFRX drives). I've never had a RAID-5 failure. Though I don't rely on RAID to keep my data safe, I think these stats do indicate that it is not even close to a certainty that you will have a second failure while rebuilding a RAID-5 system. Does it happen sometimes? Yes. Is it more likely to fail than not? In my experience clearly no.

The NV+ did fail last month btw (a PSU failure). It still had 3 months of warranty left, and I received a new PSU from Netgear which I installed yesterday. The array remained intact.

I also have had a Duo which began its life in RAID-1. That array did fail (OS partition corrupted in a unexpected power failure). As a result I purchased two UPS (and all my NAS are connected to a UPS).

I intended to rebuild the Duo as jbod, but accidentally ended up in spanning RAID-0, and left it. It failed earlier this summer (disk failure), and has been rebuilt again as jbod.

About 6 drives have failed in the various NAS over their lifetime, with a combined operation of about 54 disk-years. Four of these were 1.5 TB Seagates (and had been used in Windows systems before being put in to the NAS). One was a 2 TB Seagate, and one was a 3 TB Western Digital.

There are 16 drives currently installed in my various NAS - one Seagate and 15 Western digital. The oldest drives are about 5 years old, the newest was installed yesterday. Presently there are six 2 TB drives, eight 3 TB drives, and two 6 TB drives. Over most of the past 5 years I've had 12 disks installed.

So
(a) 9 years of operation with RAID-5, and so far no array failures and at least 6 disk replacements
(b) ~2 years of operation with RAID-1 with one failure (system related)
(c) ~2 years of operation with spanning RAID-0 with one failure (disk drive related)

(d) about 6 drives failed in various NAS, over a combined operation of about 54 disk-years. So far, this is about 1.2 failures per calendar year with 12-16 disks installed.
(e) 1 NAS failure over about 15 NAS-years of operation, which was fixed under warranty

I plan to stay with RAID-5 for the main NAS, and jbod for backups (though I'll keep the NV+ running RAID-5, probably until it fails) - keeping at least 3 copies of all data for safety. If I see a pattern of RAID-5 breakdowns here (correlated with larger drives) that could change my mind. I also expect that progress with built-in redundancy in btrfs will stabilize over the next year or so, and that will create some new options worth considering.