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Computer Manufacturers Deliberately Obscure the Longevity of Hard Disks


By Charles Miller

Every so often, I have an experience which reminds me that I might not really understand all of the things I think I understand. There are words and terms we all see frequently, sometimes use, but may not comprehend completely. Case in point is the conversation I recently had with a client who was understandably upset over having lost his valuable and irreplaceable data when a hard disk suddenly crashed without warning.

My client is a smart fellow, albeit not quick enough to back up his data, and he had read the technical specifications for his computer’s hardware. Manufacturers often publish a Mean Time Between Failures (MTBF) number, and my client knew that the number for his hard disk was 300,000 hours, which to him meant he did not need to bother backing up his data because he thought the manufacturer had promised his hard disk was guaranteed not to fail for 34 years.

Venting his frustration, the fellow peppered me with several questions: “Why did the manufacturer publish a MTBF of 300,000 hours? Why did it fail after only 20,000 hours (three years)?”

A common misconception, and one I mistakenly believed, is that MTBF was an educated guess of a component’s useful life. What I think I understand now is that MTBF is a number not applied to individual units but to an entire factory production run of many units. For example, when a factory manufacturers a million hard disks, MTBF is the sum of the lengths of the “operational periods” divided by the number of failures observed in spot tests.

Operational periods? I just learned the technical term for that is Mean Time to Failure (MTTF), and that seems to be a number hard disk manufacturers don’t want to divulge to the public. This creates a bizarre situation where manufacturers can declare a certain model of hard disk has an MTBF of a million hours (115 years) when everyone knows it would be a miracle if any hard disk lasts that long. The secret MTTF number is probably only 5–7 years.

In reality, MTBF is only useful in comparing the differences in one model of an item versus another. A high MTBF number on a hard disk means the engineering design and materials are good while the factory manufacturing the disks also achieved a high level of quality control.

It came as a surprise to me to learn that MTBF numbers are not particularly valuable in predicting when a specific model or device might fail.

The cold, hard fact is that over a period of years, the failure rate for all computer hard drives is darn near 100 percent. Eventually they almost all stop working, so please back up your important data.

Charles Miller is a freelance computer consultant, a frequent visitor to San Miguel since 1981, and now practically a full-time resident. He may be contacted at 044 415 101 8528 or email FAQ8 (at)


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