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MTBF of a Disk Drive
MTBF stands for "mean time between failures,". It is the average elapsed time that passes before a failure occurs in a batch of drives under intense test conditions.
However, a brand XYZ harddisk with a MTBF rating of 1 million hour does not mean that the average unit will run for 114 years before it fails.
In fact, MTBF of a drive is obtained by multiplying a large quantity of the drives (few hundreds or thousands) with the number of hours running before experiencing a failure in the batch.
For example, when a disk manufacturer batch tested 1500 units of hard disk and achieved an average of 30 days operation out of the batch between each individual unit failure, then the MTBF of the disk is 1500 x 30 x 24 hours = 1 million hours.
Acceleration factors that are derived from standard statistic can be used to extrapolate the value of MTBF. For example one could increase the burn-in temperature to reduce the actual test time needed to meet a required test hours.