Sharp Scheduler and the System Monitor add-on help you deal with low disk space problems in style.
In your Sharp Scheduler module management console, create a new Job and add the "Low disk Space" trigger. You can set the threshold with a value by your choosing. The trigger will fire whenever the disk space goes below the threshold and it will execute the actions you defined – this can be a Send Email Action for example, that will get you notified. As easy as 1,2,3!
You can find more information in the video below, where Bogdan explains the setup in details and will also present a solution for these kind of situations.