Backblaze has a very interesting blog on the life span/cycle of disk drives. Besides from providing a great product they also provide valuable insights into life span of disks (which fail most frequently).
Drive Failure Over Time: The Bathtub Curve Is Leaking
Hard drive failure rates tend to follow the “bathtub curve.” What is this curve, and how does it indicate the drive failure rates at Backblaze?
www.backblaze.com