A 2007 study published by
Google suggested very little correlation between failure rates and either high temperature or activity level. Indeed, the Google study indicated that "lower temperatures are associated with higher failure rates". Hard drives with S.M.A.R.T.-reported average temperatures below 27 °C (81 °F) had higher failure rates than hard drives with the highest reported average temperature of 50 °C (122 °F), failure rates at least twice as high as the optimum S.M.A.R.T.-reported temperature range of 36 °C (97 °F) to 47 °C (117 °F).
[12] The correlation between manufacturer/model and failure rate was relatively strong. Statistics in this matter are kept highly secret by most entities — Google did not relate manufacturers' names with failure rates,
[12] though they have since revealed that they use Hitachi Deskstar drives in some of their servers.
[13]
Google's 2007 study found, based on a large field sample of drives, that actual annualized failure rates (
AFRs) for individual drives ranged from 1.7% for first year drives to over 8.6% for three-year old drives.
[14] A similar 2007 study at
CMU on enterprise drives showed that measured MTBF was 3–4 times lower than the manufacturer's specification, with an estimated 3% mean AFR over 1–5 years based on replacement logs for a large sample of drives, and that hard drive failures were highly correlated in time.
[15]
