http://blog.backblaze.com/2013/11/12/how-long-do-disk-drives-last/ Here is a summary from their well written POST. ” the percentage of drives at Backblaze that are still alive at different ages: For the first 1.5 years, drives fail at 5.1% per year. For the next 1.5 years, drives fail LESS, at about 1.4% per year. After 3 years though, failures rates skyrocket to 11.8% per year.” – BackBlaze