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Harddrives have ratings for expected loss of data integrity measured as Mean Time Between Failures (MTBF.) Modern harddrives have MTBFs in the millions of hours of continuous power on. So the odds of 20 harddrives all failing at the same time is:
MTBF x MTBf x MTBF... times 17 more. In other words, as with top tier hd comanies like Western Digital with MTBFs of 1 million hours, the chances of 20 drives faling at the same time is once per life age of the Earth. Or there abouts.
Being a nerd I had to work this out. A million hours is over 114 years. So 114 x 114...is oh I was way off. Just to the 10th hd the number was older than the universe already so...![]()
In the real world MTBF for a hard drive is 0 to 7 years. Western Digital were the worst, only in the last 5 years have their drives lasted 5+ years. IBM made the good drives that last up to 7 years continuous use.
Study by Carnegie Mellon University revealed hard drive manufacturers are exaggerating their mean-time before failure (MTBF) ratings on hard drives. In fact, researchers at Carnegie indicated that on the average, failure rates were as high as 15 times the rated MTBFs.
Most layman call the computer tower itself the hard drive. When the computer fails, the entire unit is destroyed to protect from data hackers & replaced. A bad neutral power line can blow computer power supplies & hard disk in the entire building. So can lightning strike. Certain virus do also kill all hard drives infected.