Did you read Bianca's paper? I'm not sure how you can ignore the evidence of non-Poisson distributions to maintain your claim that "failures are still a Poisson process"... Also, it's *very* likely that vintage is correlated with temperature. Servers built in 2001 are very likely to have different thermal characteristics (e.g., different airflow, layout, and thus disk drive temperatures). On top of that, the power consumption of disk drives has increased meaningfully over that time period, so that a 2001 5400rpm drive very likely runs cooler than a new 7200rpm drive.