While Google looked only at desktop hard drives, Carnegie Mellon covers failure rates for both desktop Serial ATA hard drives and workstation/server-class SCSI/FC hard drives. Regarding the relationship between the two, the study says, "Interestingly, we observe little difference in replacement rates between SCSI, FC and SATA drives, potentially an indication that disk-independent factors, such as operating conditions, affect replacement rates more than component specific factors." The study also says failure rates increase in a fairly linear fashion with drive age, suggesting drives simply "wear out" over time.
Another interesting part of the study deals with RAID arrays and suggests that RAID isn't as good a prevention against data loss as some might believe. "[A] key application of the exponential assumption is in estimating the time until data loss in a RAID system. This time depends on the probability of a second disk failure during reconstruction, a process which typically lasts on the order of a few hours. The graph shows that the exponential distribution greatly underestimates the probability of a second failure during this time period. For example, the probability of seeing two drives in the cluster fail within one hour is four times larger under the real data, compared to the exponential distribution."