Another issue that affects the lifespan of an HDD is drive temperature. Variations in temperature and humidity have an impact upon drive life. Reading through the Backblaze blog, one comment that drew my eye was on server rack temperature;
... we've observed in the past three years that: 1) hard drives in pods in the top of racks run three degrees warmer on average than pods in the lower shelves; 2) drives in the center of the pod run five degrees warmer than those on the perimeter; 3) pods do not need all six fans-the drives maintain the recommended operating temperature with as few as two fans; and 4) heat doesn't correlate with drive failure (at least in the ranges seen in storage pods).
Backblaze claims that drive temperature doesn't affect drive life. That is counter to the observations of many others, including drive manufacturers. There is a reason for specified temperature ranges for HDD's. Though they are likely within these ranges, the Backblaze drives cannot be directly compared to each other with varying temperature ranges, let alone other drives. Once again, the lack of pertinent information makes any real conclusions impossible, and the uneven nature of the test environment spoils the data.
Each drive is designed meticulously to provide a tightly-defined service level in its intended environment. These guidelines determine not only the design, but also the type of components used. The most cost-effective drives are designed to deliver exactly the correct performance and longevity in their intended environment, and nothing more. Utilizing robust components above the workload requirements of the drive is wasteful, and adds cost unnecessarily. This design efficiency also means the drives are more likely to fail in untoward conditions.
Backblaze procures the cheapest possible HDD on the market at all times, regardless of its workload rating, and then subjects them to a harsh environment that is virtually guaranteed to destroy the drive. This leads to higher failure rates than observed in the wild. This reflects just how precisely these drives are engineered to fulfil their stated purpose, and nothing more.
Another concern is the direct comparisons between drives, even though they endure varying workloads. Comparing drives of the same make and model is impossible if the same workload isn't applied. Expanding that out to compare different models and manufacturers is even more ridiculous. There is no way to know how many times the drives have spun up and down, and how many times the drives were subjected to varying types of data requests.
Random data requires more movement, and thus creates more wear and tear on delicate HDD heads. Spinning up and down, and also entering and recovering from various sleep states, also wears the drives differently over the course of time. With no real rhyme or reason to the workload distribution, let alone the environment, direct comparisons are impossible.
Only one thing is certain; the drives were subjected to workloads well beyond their design limits.