For most enthusiasts, choosing a hard drive usually comes down to performance specs. This includes spindle speed, areal density, what size buffer it comes equipped with, and any special features like NCQ. But as solid-state drives (SSDs) start to trickle into the mainstream, expect to see a greater importance placed on the mean time before failure (MTBF) rating.

It'd be nice if hard drives could last forever, but like every other component that makes up your PC, hard drives eventually die. And it's never a pretty sight, either. Sometimes a HDD will give up the ghost without warning, leaving you frantically looking for ways to revive the drive long enough to extract your data (remember the freezer trick?). Other times you're given ample warning of an impending failure, typically in the form of unpleasant grinding noises, disk errors, an unusual clicking noise that wasn't present before, S.M.A.R.T. warnings, and other telltale signs that it's time to backup your data.

Most hard drives come with a MTBF rating ranging from several hundred-thousand hours to well over a million hours. At a glance, that works out to almost 200 years in some cases, but as anyone who's ever own a computer can attest, the average hard drive doesn't last nearly as long. Does that mean hard drive manufacturers are lying?

As eWeek writes, "any given MTBF is based on specific testing conditions, including obvious issues like temperature, which can affect the life of the drive." A variety of other factors also come into play, like whether or the not the manufacturer rates the hard drive based on 24/7 usage.

Despite how manufactures rate their hard drives, which may or may not be easily found by looking at a spec sheet, life expectancy remains one of the most ambiguous aspects of purchasing a HDD. Can a drive with a MTBF of 1.6 milion hours be expected to significantly outlive one rated at 750,000 hours? Good question, and one drive manufacturers will have to start addressing in more meaningful terms as engineers continue to push the envelope, and flash memory with limited write capablities start to become more prevalent.

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