I encourage mac users to use the free hard drive monitoring utility called SMARTReporter. It monitors the SMART telemetry from modern hard drives and will alert you to an imminent failure. I'm sharing this because I just got my first such alert on one of my home systems.
SMARTReporter is available here.
Here's a screencap of the alert that I received.
I started using this utility after reading a Google research paper published 2 years ago that was based on their extensive experience with huge numbers of hard drives. Here's a link to the Google paper.
A quote from the conclusions: "We find, for example, that after their first scan error, drives are 39 times more likely to fail within 60 days than drives with no such errors."
For the curious, here's the Wikipedia page on SMART.
After dismissing the warning alert, I started Disk Utility to see what it would tell me. It reported that the suspect drive was failing and did not allow me to try "disk repair" or anything else. Here are a few screencaps of that.
Disk Utility does not have an agent running int he background that can alert the user like the SMARTReporter does. That's unfortunate, and I hope future versions of Mac OS will include that capability.
Since the drive was still usable, I quickly copied all files to a healthy drive. A few days later I bought a replacement SATA drive at Fry's and it's now installed and humming away happily.
I also discovered a comparable free utility for Windows users called Active Hard Disk Monitor.