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MTBF is a population statistic, not a lifespan: 1.4 million hours does not mean a drive lasts 160 years, it means about 0.6 percent of a population fails per year of continuous operation. Multiplying by fleet size gives the expected replacements to plan for, and holding roughly 1.5 times that as spares covers clustering.
Failure rate from MTBF
AFR = 1 - exp(-annual power-on hours / MTBF); expected failures = drives x (1 - exp(-total hours / MTBF))
Often not. Large-scale field studies report annual failure rates of 2 to 5 percent against ratings implying under 1 percent, partly because MTBF excludes infant mortality and end-of-life wear.
Failure rates are high initially from manufacturing defects, low and flat through mid-life, then rise as mechanical wear accumulates. A constant AFR only describes the flat middle.