Log in to CMC and go to Monitoring page. Go to Watchlist tab and click New.
Give a name for your watch and select "Two (Ok, Danger)". Click Next.
Type "disk" as filter and choose "Free Disk Space in Root Directory". Change the value of Danger. In this example i set it to less or equal then 100 GB.
Click Next. At the Bottom, set the notification settings. Save your watch. You can add also e-mail alerts. Now copy a large file to your disk for test the watch. If your free disk space becomes less then 100 GB, you will get an alert depending on your notification settings.
