Full definition
Manufacturers specify MTBF in millions of hours (e.g., 1,500,000h = 171 years for one specific disk) but it is a population statistic, NOT a prediction for an individual disk. AFR is more useful for planning: typical enterprise HDD has 0.35% to 1% AFR; enterprise SSD ~0.3% to 0.5%. Backblaze public data (quarterly report) is a reliable reference. MTBF and AFR do not replace backup. Even with 0.5% AFR, in a 200-disk fleet one failure per year is expected. In RAID 6 systems with same-batch disks, correlated failure (same firmware defect or wear) often occurs within short windows. Immutable backup is the only protection against statistically inevitable failure.
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