Did anyone recognise the brand/model of HDD they use? Would be interesting to know what Google chose as I'm sure they spent a lot of time/money researching the best HDD for their needs.
If Steven Levy's "In the Plex" reflects current reality, Google buys very cheap failure-prone HDDs and builds in a lot of redundancy: "Google's first CIO, Douglas Merrill, once noted that the disk drives Google purchased were 'poorer quality than what you would put into your kid's computer at home.'" [page 183]
I seem to remember a paper discussing failure rates, and that there wasn't a large statistical difference between the 'poorer quality' hard drives and the higher end stuff.