It's always a little unnerving to read user reviews while shopping for a new hard drive. No matter how well-reviewed a drive is, on average, there are still those scary exceptions--reports of drives that died a day or week or month after they arrived, still practically brand new. Backblaze, an online backup site, calls those infant mortality drive failures. Those kinds of hard drive deaths represent just one statistic in a huge pile of data Backblaze has collected on hard drive longevity, which the company has used to try to answer a very difficult question: How long do hard drives last?
Because Backblaze has only been around for five years, it doesn't have a conclusive answer quite yet. But it has enough data to make an educated guess, based on the near-28,000 hard drives (providing 75 petabytes of cloud storage) the company has installed since 2009. All but six petabytes of that data is stored on standard internal hard drives, while the rest is stored on hard drives ripped from external shells--Backblaze had to turn to those drives during the shortage created by Thailand's flooding crisis.
So here's the good news: Many hard drives in operation at Backblaze have been around as long as the company, giving them lifetimes of more than five years. At home, you shouldn't have to worry too much about replacing drives every couple years. But the data is a bit more nuanced and interesting than that.
"Reliability engineers use something called the Bathtub Curve to describe expected failure rates," Backblaze's blog explains. "The idea is that defects come from three factors: (1) factory defects, resulting in 'infant mortality', (2) random failures, and (3) parts that wear out, resulting in failures after much use."
For the first 18 months a drive is in operation, its failure rate hovers around five percent. That means five percent of drives end to fail during their first year and a half of operation. After a year and a half, however--once factory defects have cleared out--the failure rate drops to an annual 1.4 percent. Then, after the three year mark, the failure rate begins to climb up to 11.8 percent. That's when drives begin to give out.
Backblaze's explanation of annual failure rates is also a key piece of the puzzles. It explains that 100 percent isn't the worst possible failure rate, but that's not the case: "Imagine you have a disk drive supplier who provides drives that are 100% reliable for six months, but then all fail at that point. What’s the annual failure rate? If you have to keep 100 drives running at all times, you’ll have to replace the drive in every slot twice a year. That means that you’ll have to replace 200 drives each year, which makes your annual failure rate 200%. So, in theory at least, there is no worst possible failure rate. If every drive failed after one hour of use, the annual failure rate would be 876,000%. Fortunately, the drives that Backblaze gets are more reliable than that."
Despite having its drives running 24/7, more than 80 percent of Backblaze's purchased hard drives are still operating. That's pretty good. Based on their data, they project that the median lifespan of a hard drive will be over six years. So even though failures become significantly more common after three years, the odds are in your favor until you hit that six year mark. After that, it's probably time to start thinking about buying a new hard drive.