"I've either personally owned or purchased for companies I've worked for dozens of hard drives "
Bwahaha... no offence mr low uid, but "dozens" does not make you an expert. I have had thousands of bad drives in my 15 or so years in the business. out of tens to hundreds of thousands of machines. Right now I see about 50-100 bad drives a year supporting a medium sized company as well as the occasional home computer.
Hard drives power supplies and capacitors are the number 1 2 and 3 things that fail on all computers first. Any other failure is an oddity.
3-5 years is most certainly correct as the AVERAGE lifetime of a drive. I have come to my own conclusions about that particular number based on experience, and reading about hard drive failures for many years. Sure many drives last longer, and many many fail by simply getting slower with age. But the main point is that dozens is merely a hobbyist and if you are basing your opinion on such a small sample size, you may not be correct.
As I said before, seagate screwed themselves over bad firmwares a few years ago which is why people hate them so much now (me included). However I am sure WD will fuck up eventually too and ill have to switch back to seagates.
The most important thing for drives is proper cooling and avoiding manufacturing defects (bad batch).
The best tool to use for hard drive diagnosis is 1) your ear and 2) hdtune