The Tech Report has linked to a .pdf article on the failure rate of hard drives.  Not many independent studies have been done, so it is hard to say how accurate manufacturers MTBF stats are.  Google ought to know as they use an enormous amount of HDDs and they are constantly running, and they are the ones who did the research.
“Disappointingly, Google omits to mention what might be the most important piece of information of all: which manufacturers have the most failure-prone drives. Perhaps the search giant doesn’t want a lawsuit on its hands, or perhaps it doesn’t want to risk compromising any juicy discounts it might receive from hard drive makers. Nevertheless, Google claims differences in failure rates between drive models or brands are not significant. “In contrast to age-related results, we note that all results shown in the rest of the paper are not affected significantly by the population mix,” the paper says.”

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