MTBF is a dimensionless number that indicates the time in hours that elapses before the next failure. For example, an MTBF value of 10,000 indicates that the device will operate for an average of 10,000 hours without failure. The MTBF is of particular importance for disk drives and hard disk drives.
There are no prescribed guidelines for the MTBF according to which the manufacturers specify it. They are theoretical values from the manufacturer that do not have to be substantiated by practical measurement results. They merely provide an indication of the robustness of the devices.
The MTBF times are determined in statistical tests in which several thousand devices are operated simultaneously over several weeks. The MTBF is then calculated from the failures. Since the values are often in the lower per thousand range, the MTBF value is converted into the annual failure rate ( AFR). An MTBF value of 100,000, which means one failure in 100,000 hours, corresponds to an AFR value of 8.76%, which means that one out of 11 drives fails on average every year.