From BBC News –
“The impact of heavy use and high temperatures on hard disk drive failure may be overstated, says a report by three Google engineers. The report examined 100,000 commercial hard drives, ranging from 80GB to 400GB in capacity, used at Google since 2001.”
Google has found that, in their sample data, drives failed more frequently at lower temperatures, than drives at higher ones, unless the drive was older (3 years or more in this case). It also found that drives which are used frequently are less prone to failure, compared with drives which see intermittent activity, even if the drives are the same age.
You can also read the original report from Google.