Of note, assuming that decommissioning of drives is driven primarily by e.g. space concerns rather than signs of impending individual drive failures (which seems to be the case based on the linked article about storage scaling), you could conduct a survival analysis in which decommissioned drives are treated as right-censored to get a better measure of the failure rate over time as well as how that failure rate depends on various factors. Note that the most common choice of a proportional hazards model may not be appropriate here, and an accelerated failure time model may be more appropriate, although I couldn't say for sure without actually working with the data.