On the Reliability of Hard Disks:  Summarizing research papers: "there was no correlation between disk failure rates and utilization, environmental conditions such as temperature, or age. This means that high disk utilization or age of the disk have no significant impact on the probability that it will fail.  .. the expected remaining time until the next disk failure grows with the time it has been since the last failure...

They did find a strong correlation between manufacturer/model and failure rates. They observed that older disks had a much lower failure rates then newer disks, where the newer disks in general were less expensive. Basically you get what you pay when you talk about disk reliability. ..

The only exception to the lack of correlation was that infant mortality rate for disks showed a correlation with high utilization: if a new disk is really crappy you can detect this by putting a high load on it. ..

Both papers report disk failure rates in the 6%-10% range: in a datacenter with about 100,000 disks you will need to replace up to between 6,000 and 10,000 disks per year. "
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