What is the oldest disk you are running/have seen in working condition in relation to S.M.A.R.T attribute number 9 = Power_On_Hours?   I have a 300GB Seagate Barracuda 7200rpm (SATA) disk here with power on hours = 57711  .. Thats over 6,5 years in runtime!     This disk has been running in a file server in a RAID5 setup (4disks) for quite some time. It has now been decommissioned by me.....