A study by Carnegie Mellon University, where the researchers took roughly 100,000 hard drives from a variety of manufacturers, concluded that hard drive manufacturers were exaggerating their mean-time before failure (MTBF) ratings on hard drives – failure rates were, on average, 15 times the rated MTBFs. The drives were tested in various operating conditions, including real world scenarios. Researchers found that drive operating temperatures had little to no effect on failure rates: a cool hard drive survived no longer than one running hot. The types of drives used in the study ranged from Serial ATA drives, SCSI and even high-end fiber-channel (FC) drives. Carnegie researchers found that high-end drives did not outlast their mainstream counterparts: "In our data sets, the replacement rates of SATA disks are not worse than the replacement rates of SCSI or FC disks. This may indicate that disk-independent factors, such as operating conditions, usage and environmental factors affect replacement rates more than component specific factors."
The study also found that the number one cause of drive failures was age. Drives tended to start showing signs of failure after roughly five to seven years of service, after which there was a significant increase in average failure rates (AFR). The failure rates of drives that were in their first year of service or shorter was just as high as those after the seven year mark. The average replacement rate of drives ranged from 2-13% annually, indicating that there is a need for manufacturers to re-evaluate the way a MTBF rating is generated. Worst of all, these rates were for drives with MTBF ratings between 1 million and 1.5 million hours. Carnegie researchers concluded that backup measures are a necessity with critically important data, no matter what kind of hard drive is being used.