I know this has been thought of and there are even existing feature requests, and may even be in the works, but I feel this needs more attention, and perhaps my level of detail will help.
Having a global settings (default) is great, but we need the capability to fine tune the node to specific settings for each hard drive and memory resource.
The easiest way that I could think to do this, is by adding a Thresholds button to the Management panel of the node.
With that button the user is prompted to Enable the Node Specific Settings
After the setting has been Enabled the user can go to the Disk Volumes and modify the default thresholds for the specific object.
In the Node Thresholds section the user will then be able to modify the thresholds for CPU, Disk, Memory and Response Time. An additional option for Disk thresholds will be added allowing the user to specify (in MB) the minimum amount free before alerting. This would be available for EACH object, and can be different between each object. The option of % Used versus Amount of MB Free is determined by the checkmark next to each option. If one is checked the other is unchecked. If neither are checked the default of % Used will apply..
Here are some of the other Feature Requests, please be sure to vote for them as well.