Focus is on disk farms. I think that in SOHO/SMB NASes, theft, human error, file system corruption, are the key risks - moreso than drive failure. This, after the initial new-drive failure time is past.http://storagemojo.com/2007/02/26/netapp-weighs-in-on-disks/
Interesting read - not the blog post - but the netapp discussion within it - (NetAPP builds big enterprise grade SAN's, and they have a lot of data on drives, RAID, and failure modes)