CloudRadar comes with a default rule that alerts you if any disk on any server has less than 10% free. If this doesn't fit your needs, create additional rules per disk.
Select the host the disks are attached to and navigate to the latest data.
Unfold the metrics section and look for the disk you want to set an alert on.Click on the wand icon. That starts the rule wizard.
Now enter the desired threshold and select if you want to create an alert or a warning. Finally add the new rule.
The rule appears in the rules section. You can change the threshold later if you like.