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[ The PC Guide | Systems and Components Reference Guide | Hard Disk Drives | Hard Disk Performance, Quality and Reliability | Hard Disk Quality and Reliability | Hard Disk Quality and Reliability Specifications ]

Service Life

While every product has a life expectancy, not every industry acknowledges this up front! Due to the incredibly precise nature of the technology used in hard disks (we're talking about millionths of an inch in many regards) and the fact that mechanical components are used, it is impossible to guarantee the reliability of even the highest-quality disks for more than a few years. Hard disks follow the so-called bathtub curve model of failures: they have a relatively high rate of "infant mortality" (or early failure), a period of very low failures for several years, and then a gradual reduction in reliability as they reach the end of their useful life. To ensure that those who rely on hard drives do not continue to use them as they enter the period of increased failure that results from component wear-out, every hard disk has defined as one of its specifications the manufacturer's intended service life for the product.

The service life of a modern hard disk is usually about three to five years. In my opinion, what the manufacturer is telling you with this number is this: "If your drive gives you good service for this length of time without any trouble, you've gotten good value from it and you should consider getting a new drive if reliability is important to you; the drive isn't going to fail the instant it hits this age, but the chances of it failing will increase significantly the longer you use it past this point". This number is in many ways more important than the "impressive" MTBF numbers that are so eagerly thrown around, and is in fact intended to be used in conjunction with MTBF to present a realistic picture of drive life and reliability.

Interestingly, the claimed service life is often longer than the warranty period for the drive. For example, the service life might be five years but the warranty period only three years. Think about what this means. ;^) Basically, it says that the manufacturer thinks the drive should last five years, but they aren't going to bet on it lasting more than three! I personally think the warranty period is a better indication of a drive's true useful life--to a point--because the warranty period is where the manufacturer "puts their money where their mouth is". Sometimes warranty periods are made longer for marketing reasons, but no manufacturer will warrant a product for three years if they expect to see significant problems in that time, because they will lose their shirts.

If reliability is important, you may want to make sure you get a new hard disk before your warranty period is up, or at the very least, before the drive reaches the end of its service life. Of course, at the rate technology changes, in three years you will probably want a new disk for performance reasons as well. Oh, and it goes without saying that anyone who cares about hard disk reliability should be performing regular backups.

The real world life of a hard disk is almost always going to be significantly higher than the stated service life, which tends to be very conservative. One reasons why is that even if the warranty doesn't cover the entire stated service life, most companies don't want to see their drives failing within the period of time that they say they won't--it looks bad. Another reason is because the service life only represents when the odds of failure increase; there is still a statistical distribution of actual failures. I have used many hard disks that have been in operation for 10 or even 12 years, and these are older technology drives that never had anywhere near the stated reliability levels of today's disks. So there certainly is no reason to believe that the day after your drive's third anniversary, you need to yank it and replace it with a new one. But just remember that the longer you go past the drive's service life, the more the chances of a failure increase. The environmental conditions, and how the drive is used, will also have an important impact on its overall life. See this section for more on component life.

Next: Start/Stop Cycles

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