Backblaze’s study finds that both AFR and MTBF are bunk. The document finds that disks follow the predicted “bathtub” curve of failure: lots of early failures due to manufacturing errors, a slow decline in failure rates to a shallow bottom and then a steep increase in failure rates as drives age.Backblaze’s disk longevity study shows something pretty close to the ‘bathtub’ curve one would expect
The study then looked at when drives fail and found a drive that survives the 5.1 per cent AFR of its first 18 months under load will then only fail 1.4 per cent of the time in the next year and half. After that, things get nasty: in year three a surviving disk has an 11.8 per cent AFR. That still leaves over 80 per cent of drives alive and whirring after four years, a decent outcome.
The study also predicts accelerated failure rates in years four and five, guesstimating things will get very, very bad in years four and five.

http://www.theregister.co.uk/2013/11/12/server_server_in_the_rack_whens_my_disk_drive_going_to_crack/