Back in January, we wrote a story about which company makes the most reliable hard drives. At the time, Hitachi came out ahead of Western Digital and Seagate by a significant margin. Now, with new data from our friends at Backblaze who have 34,881 hard drives spinning their little hearts out as you read this sentence, we’re going to try and investigate the matter of why there’s so much variation between the reliability of the big three hard drive makers.

New data on hard drive reliability

First up, let’s run through Backblaze’s new figures, and try to weed out any erroneous or untrustworthy data that shouldn’t be used to draw any hasty conclusions. Backblaze produced its first set of drive reliability figures at the end of 2013, when it had 27,134 hard drives plugged in. This new set of data, produced at the end of June 2014, tracks how those original hard drives are doing (some of which are now four years old), and the reliability of some newer drives (mostly the 4TB Seagate Desktop HDD.15 and HGST Megascale 4000.B.

The gray bars show the annual failure rates at the end of 2013; the colored bars show the updated annual failure rates as of June 2014. “Annual failure rate” is a slightly odd statistic, but it’s a good way of measuring a large number of drives that come from different manufacturing batches, different vendors, and are of a different age (some of the drives are four years old; some are just a few weeks old). In short, the annual failure rate is the percentage chance of a hard drive dying in a given 12-month window.

The annual failure rate is averaged over a drive’s entire life, though this second graph shows that most hard drives have a pretty steady decline over the first three years, after a quick drop-off at the start to account for lemons.

What this graph primarily shows us it that, if you’re running a Seagate drive that’s more than three years old, you should probably consider backing up your data and getting a new drive.

What this graph doesn’t show us is how reliable Seagate, Western Digital, and HGST’s newer hard drives are. It’s also worth noting that since Backblaze started gathering data, Seagate has acquired Samsung and Western Digital has acquired HGST — and there’s no way of telling how intermingled the technologies and manufacturing processes are.

Read: The 270TB storage pod: Does Seagate or WD make the ultimate 6TB hard drive?

As far as helping you pick out who is currently making the best hard drives — as in, what hard drive should you buy today — Backblaze provides the following table:

You can see that Backblaze is using three new hard drives: Western Digital Red 3TB, Seagate Desktop HDD.15 4TB, and HGST Megascale 4000 4TB. So far, it isn’t looking very good for the Western Digital Red — an 8.8% annual failure rate in the first year is very bad. The Seagate drive is doing OK. The HGST drives (which is a low-end enterprise-class hard drive) hasn’t had enough time or failures yet (but it’ll probably be very good, judging by the other HGST/Hitachi drives).

Overall, though, the average lifespan of hard drives at Backblaze is still generally very high. No matter which brand you go for, there’s a very good chance that your hard drive will last longer than four years — especially if it’s made by HGST.

Why are some hard drives more reliable than others?

Putting aside any issues of pertaining to scientific rigor (or lack thereof) behind Backblaze’s data, it’s clear that there’s a big variation in hard drive reliability — and it’s also clear that HGST’s drives are much, much more reliable than anything else in the consumer hard drive market. (The Megascale drives are enterprise-class, but the thousands of Deskstar drives are consumer-grade).

Unfortunately, because there isn’t a scientifically-gathered data set for modern hard drive reliability. This is why, no matter which drive you search for on the internet, you’ll always find a forum post decrying its poor reliability. The best we can do, then, is to offer up some educated guesses.

Manufacturing defects. Historically, hard drives usually failed in batches. Maybe it was a bad batch of motors, or the usual QA guy was off sick.

Bad design. Sometimes, one hard drive is simply designed better than the others. Maybe it puts less stress on the motor or read/write head arms. The infamous IBM “Deathstar” (Deskstar) probably had bad reliability due to bad design — or perhaps just a very unlucky streak of manufacturing defects.

Rebadged enterprise drives? One theory behind Hitachi/HGST’s incredible reliability is that the Deskstar is actually an enterprise-class hard drive, but sold as a consumer-level drive. Enterprise drives are generally made to a higher standard and undergo more strenuous quality assurance.

Damaged goods. Another possibility, especially if you buy all of your drives from the same third-party vendor, is that all of the drives may have been damaged in some way. Maybe they were dropped. Or maybe it was a shady retailer re-selling old drives. (For what it’s worth, Backblaze says it obtains its drives from a variety of vendors, and during the Thailand floods and hard drive crisis it even farmed hard drives from external USB enclosures.)

Better (or different) technology. Of course, some hard drive makers are simply better than others. Western Digital/HGST and Seagate have been making hard drives for decades, and both have their own intellectual property portfolios and their own ways of making hard drives. There are intrinsic differences in their hard drives that can make one drive more reliable than the other. Both companies will aggressively reverse-engineer the opposition’s drives to narrow the larger gaps, but small differences will always exist.

In short, there are a variety of reasons for hard drives to have variable reliability. Our previous recommendation was to buy a Western Digital/HGST drive — but the high failure rate of Western Digital Red 3TB drives is a little worrying. As long as you perform regular backups, you should be fairly happy with the reliability of most modern hard drives.

Now read: SanDisk’s collosal 4TB SSD: Does this mean SSDs will soon provide more storage than hard drives?