Is there an app that can check hard disk health periodically, or an app that just runs in the background that will notify the user that the disk is nearing the end of its useful life?
One way would be using smartmontools. I know this is not a unity app but a commandline tool, but it show you the disk health right now.
You can install it using
sudo apt-get install smartmontools installed it with apt-get
you can run it using
sudo smartctl -H /dev/sda
This will run a test on the drive sda and print the results.
I hope it helps just a little :)