When you shop for a drive, you might notice a statistic called the mean time between failures (MTBF) described in the drive specifications. MTBF figures usually range from 300,000 to 1,000,000 hours or more. I usually ignore these figures because they are derived theoretically.

In understanding the MTBF claims, you must understand how the manufacturers arrive at them and what they mean. Most manufacturers have a long history of building drives, and their drives have seen millions of hours of cumulative use.

They can look at the failure rate for previous drive models with the same components and calculate a failure rate for a new drive based on the components used to build the drive assembly. For the electronic circuit board, they also can use industry-standard techniques for predicting the failure of the integrated electronics.

This enables them to calculate the predicted failure rate for the entire drive unit. To understand what these numbers mean, you must know that the MTBF claims apply to a population of drives, not an individual drive.

This means that if a drive claims to have an MTBF of 500,000 hours, you can expect a failure in that population of drives in 500,000 hours of total running time. If 1,000,000 drives of this model are in service and all 1,000,000 are running simultaneously, you can expect one failure out of this entire population every half-hour.

MTBF statistics are not useful for predicting the failure of any individual drive or a small sample of drives. You also need to understand the meaning of the word failure. In this sense, a failure is a fault that requires the drive to be returned to the manufacturer for repair, not an occasional failure to read or write a file correctly.

Finally, as some drive manufacturers point out, this measure of MTBF should really be called mean time to first failure. "Between failures" implies that the drive fails, is returned for repair, and then at some point fails again. The interval between repair and the second failure here would be the MTBF.

Because in most cases, a failed hard drive that would need manufacturer repair is replaced rather than repaired, the whole MTBF concept is misnamed. The bottom line is that I do not really place much emphasis on MTBF figures. For an individual drive, they are not accurate predictors of reliability.

However, if you are an information systems manager considering the purchase of thousands of PCs or drives per year or a system vendor building and supporting thousands of systems, it is worth your while to examine these numbers and study the methods used to calculate them by each vendor.

If you can understand the vendor's calculations and compare the actual reliability of a large sample of drives, you can purchase more reliable drives and save time and money in service and support.