Werner Vogels has a really interesting post up about the reliability of hard disks. The bottom line? You get what you pay for.

In their study they found that 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. 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. Given that disks in general arrive in large batches you may want to take care with how you deploy these disks as you want to reduce the impact of these strong failure correlations.