MTBF
Mean Time Between Failures — a statistical measure of how long a device is expected to operate between failures. Drive manufacturers quote 1-2 million hours (~150 years), which is misleading — it's a window estimate from fleet testing, not a guarantee.
Calculation: run 1000 drives for 1000 hours = 1 million drive-hours; if 2 failures occur → MTBF = 500,000 h. Says nothing about whether an individual drive lasts that long. AFR (Annualized Failure Rate) is more useful: %-failure/year. Backblaze publishes real AFR data per drive model (~1-2% typical, worst models 5%+). MTTR (Mean Time To Repair) is the complement for system-availability calculation: A = MTBF / (MTBF + MTTR).