I've heard that the accepted wisdom is that you should replace hard drives every three years because the older they get the more likely they are to fail. I've never actually followed that advice because I keep daily backups so I figure they should run until they die. What's the longest you've ever had a hard drive running continuously? I have 4 1TB Western Digital HDs (WDC WD1001FALS-00J781) running in a Mac Pro 2008 nearly continuously (24/7/365) since 2009, other than for updates that require a shutdown or a reboot and the rare power outage. The Mac Pro has been hooked to various APC Smart-UPS over the years, currently a 1500VA. So far they all show no errors when checked with SMART Utility (using both the short and long tests) and have over 78000 hours on them. Back in the day I tested them with SpinRite before putting them into production to make sure they didn't have any bad sectors. Nowadays I use the disk certification feature in SoftRAID to do the same for drives I use with my other Macs.