Hi,
You can locate the monitor and override it based on your requirements:
How to Monitor Using Overrides
http://technet.microsoft.com/en-us/library/bb309719.aspx
Best practices to use when you configure overrides in System Center Operations Manager 2007
http://support.microsoft.com/kb/943239
For the Logical Disk Free Space Monitor, I would like to share the following post with you for your reference:
Logical Disk Free Space Monitor
http://blogs.technet.com/b/jonathanalmquist/archive/2009/04/04/logical-disk-free-space-monitor.aspx
Hope this helps.
Thanks.
Nicholas Li
TechNet Community Support
