This from an NEC white paper in 2008:
"A recent academic study [1] of 1.5 million HDDs in the NetApp database over a 32 month period found that 8.5% of SATA disks develop silent corruption. Some disk arrays run a background process to verify that the data and RAID parity match, a process which can catch these kinds of errors. However, the study also found that 13% of the errors are missed by the background verification process. When you put those statistics together, you find on average that 1 in 90 SATA drives will experience silent data corruption not caught by the background verification process. So when those data blocks are read, the data returned to the application would be corrupt, but nobody would know. For a RAID-5 (4+P) configuration at 930 GB usable per 1 TB SATA drive, that calculates to an undetected error for every 67 TB of data, or 15 errors for every petabyte of data. If a system were constantly reading all that data at 200 MB/sec, it would encounter an error in less than 100 hours."
Sometimes, I just want to weep.