Garth Gibson, associate professor of computer science at Carnegie Mellon and co-author of the study, was careful to point out that the study didn't necessarily track actual drive failures, but cases in which a customer decided a drive had failed and needed replacement. He also said he has no vendor-specific failure information, and that his goal is not "choosing the best and the worst vendors" but to help them to improve drive design and testing.

He echoed storage vendors and analysts in pointing out that as many as half of the drives returned to vendors actually work fine and may have failed for any reason, such as a harsh environment at the customer site and intensive, random read/write operations that cause premature wear to the mechanical components in the drive.

Several drive vendors declined to be interviewed. "The conditions that surround true drive failures are complicated and require a detailed failure analysis to determine what the failure mechanisms were," said a spokesperson for Seagate Technology in Scotts Valley, Calif., in an e-mail. "It is important to not only understand the kind of drive being used, but the system or environment in which it was placed and its workload."

"Regarding various reliability rate questions, it's difficult to provide generalities," said a spokesperson for Hitachi Global Storage Technologies in San Jose, in an e-mail. "We work with each of our customers on an individual basis within their specific environments, and the resulting data is confidential."

Ashish Nadkarni, a principal consultant at GlassHouse Technologies Inc., a storage services provider in Framingham, Mass., said he isn't surprised by the comparatively high replacement rates because of the difference between the "clean room" environment in which vendors test and the heat, dust, noise or vibrations in an actual data center.