Hard drives are the heart of our digital lives. They house all of our precious data. When they fail, as they inevitably will, the results can be catastrophic. That’s why it’s important to have an accurate understanding of just how long a given hard drive is likely to last — the better to anticipate the moment when it will fail.

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While there’s no simple formula for knowing just when a drive will fail, a new study from the online backup service Backblaze has helped shed some light on just how long hard drives last.

To keep their cloud service up and running, Backblaze has used over 25,000 “consumer-grade” hard disc drives in their data center. That gives them a fairly large set of drives to test. What they’ve found: 26 percent of those drives failed in the first year while 74 percent of them are still up and running after four years of continual use.

It’s worth reading Backblaze’s study (linked above) for the full technical details, but the shorter version is that they found that the failure rate of hard drives tends to increase after year three. The company estimates that the median lifespan for a hard drive is probably six years (meaning that half of all hard drives will fail before the six year mark, half will fail after).

One critical piece of hard drive survival is how they’re used. Backblaze is a cloud storage provider, so their drives are in constant use as consumers upload and back-up their data. As their study notes, the failures that generally impact drives over the first two years tend to be things like faulty construction or a random issue, but beyond that hard drives (which consist of moving parts) simply start to break down from repeated use.

That means you’re likely to enjoy more than the median life span of six years if you use an external hard drive that’s mostly disconnected and powered off when not in use.

Backblaze has promised to release data quarterly so it can form an even more accurate picture of hard drive performance over time. Hopefully others will do the same. In the meantime, if you have a hard drive coming up on the three or six year mark, be sure all of its contents are backed up either on another drive or in the cloud (or, ideally, both). Just in case.