I've worked in IT for several years. My own fail rate is one or two drives per year in a 50 computer environment. So in a 150 computer environment its 3 to 6 a year. That's desktops/laptops.

In servers its a completely different game. Thanks to A/C and steady loads its much, much better. There is a chance of getting a run of bad disks and suddenly having multiple failures a year, but only on that specific model. Generally, I'd say that rate is closer to .25-.5 fails for every 50 drives per year, if that. So over 4 years I can expect one or two drive fails on 50 disks.

Regardless, drives fail all the time on desktops and laptops. The reliability is a huge, huge problem. Supposedly, SSDs were going to fix this, but their teething problems are probably making them worse than spinning disks.

It stands to reason that people would think that hard drives are the least reliable component. A bad hdd tends to be very obvious, and for anyone with bad backup hygiene also memorable. Something like a flaky ram stick just makes the machine crash a little more often.

That said, on my desktop I run a raid-1 setup. When a drive fails, I immediately replace the whole pair. Just lost the third one in 6 years this Saturday. And there isn't even any infant mortality distorting the stats.

Completely anecdotally, I've had a few hard drives fail (some spontaneous, some due to physical abuse that solid state hardware would tolerate) and never seen a power supply go. I suspect that power supplies are something where cheap ones will fail a lot and good, expensive ones will outlive the person buying it. Hard drives, on the other hand, have a decent risk of failure no matter how much money you spend.