MTBF is a measure of the average time it takes for a hard drive to fail. MTBF is important because it can help you determine how often your hard drive is failing and how long it will take for your data to be lost. MTBF can also help you determine whether you need to replace your hard drive.

What Is MTBF?

Simply put, MTBF, or Mean Time Between Failures, is the estimated time it will take for a device to fail. Depending on who you ask (or which company is manufacturing the product), there are a few different ways to calculate MTBF. At its core, calculating MTBF is a simple process of analyzing how long a hard drive, SSD, or other product has been running and averaging that with the number of times it has failed. This will give you the mean time between failures.

For example, a product that runs for 24 hours before encountering an issue would get an MTBF rating of 24 hours. A product that experiences 2 failures in 24 hours would get an MTBF rating of 12 (that is, 24 hours / 2 failures = 12).

Of course, this becomes slightly more complex when you throw in extended runtimes and multiple failures, but the core concept remains the same.

Why Does MTBF Matter?

If you’re looking to buy a new hard drive, MTBF is one of the most important factors to consider. Hard drives are meant to be used as long-term storage solutions—allowing you to access your files and data years down the line. Because of this, you’ll want to buy products that offer large MTBF values. Products that can go a long time without encountering a failure are more likely to preserve your files than ones with a low MTBF value.

Encountering a failure doesn’t necessarily mean your drive is completely useless and your files are lost. However, you’ll likely need to go through an extensive recovery process or consult a professional to help retrieve anything on the drive. It’s also a headache that’s (almost) entirely avoidable if you purchase the right hard drive.

Use MTBF to Make Smart Purchases

Unfortunately, MTBF ratings aren’t foolproof. Many manufacturers calculate their MTBF values by running a drive for thousands of hours straight and calculating when they finally fail. But that’s not necessarily representative of how you’ll use the drive in your home. Many people turn their drives on and off at regular intervals, uninstall and reinstall them, or simply keep them stored in a room that’s outside the optimal temperature for peak performance, all of which could have an impact on how accurate the listed MTBF value is.

Because of the discrepancy in how drives are tested compared to how they are used, it can be difficult to figure out a true MTBF. Throw in the fact that some manufacturers might use slightly different formulas for computing MTBF, and it’s clear that MTBF values shouldn’t be taken at face value.

Despite its shortcomings, MTBF is one of the best ways to gauge the reliability of your hard drive. Another safe bet is to buy products from trusted brands such as Western Digital and Seagate. These manufacturers have a reputation for building reliable products that should last for years before running into problems.

If you need something truly reliable, stick to products from trusted brands that offer an MTBF rating over one million hours like Seagate’s IronWolf Pro. Again, this doesn’t mean your drive will run for one million hours before failing (rather, it’s an average number based on the manufacturer’s testing), but a large number like this should give you some peace of mind when making a purchase.

If you’re only going to be using your drive for short-term storage, then you can get away with something smaller. We’d recommend going with the most reliable product you can afford. But if you’re on a budget and will only be using your drive to transfer files quickly, then something cheaper with a lesser MTBF could pose a good value, like the WD Red Plus.

Not sure where to start? Be sure to consult our guide highlighting the best external hard drives. And if you find a good-looking product that doesn’t advertise its MTBF, try searching for its spec sheet from the manufacturer. Alternatively, you can look for its warranty length—giving you an idea as to how long the drive will last before you should start looking for a replacement.

What About AFR?

The story of hard drive reliability doesn’t end with MTBF. While many companies are still using this value to predict reliability, Seagate will sometime use Annualized Failure Rate (AFR) for its products. Again, this is exactly what it sounds like — the probability that a device will fail during a year of use. Unlike MTBF, you’ll want this to be as small as possible. The most reliable drives offer an AFR below 1% and should last you a remarkably long time before needing a repair or replacement.

Just like with MTBF, remember that AFR is not giving you individual performance indicators but is an aggregated value based on the controlled testing of numerous products. Your drive’s performance may vary from what’s advertised, but AFR (like MTBF) is a great starting place when looking for reliability.

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