A study of about 100,000 drives conducted by Carnegie Mellon University proves that vendors are not telling the truth about the reliability of disk drives.

The study says that customers are replacing disk drives at rates far higher than those suggested by the estimated mean time between failure (MTBF) supplied by drive vendors.
 

Fibre Channel (FC) drives are just as reliable than less expensive but slower performing Serial ATA (SATA) drives, the report says. The Carnegie Mellon study examined large production systems, including high-performance computing sites and Internet services sites running SCSI, FC and SATA drives.

The figures shed that annual failure rates were between two and four percent, "and up to 13 percent observed on some systems”. 

However data sheets from vendors show that that the failure rates should be only  0.88 percent." However, the study showed typical annual replacement. You can find more details here.