Detect failing USB hard drive automatically

I Have a Raspberry Pi which has one USB exteal hard drive attached to it (via a separatly powered USB hub). The thing is that sometimes this disk becomes unresponsive (I can't access its mounted directories any more). A quick fix I found for this is just to restart the Pi.

The Pi is ruing a minor home server so it would be nice to have it always online, but 100% availability isn't so important. What's more important for me is that I don't have to think about it or tinker with it every so often.

So... My goal is to automate this. Currently my idea is to have a cron job ruing every few minutes to detect the unresponsive disk and if so, to reboot the Pi. The only thing I'm not sure about is how to detect when it becomes unresponsive, what's the best approach.