When monitoring Windows Server 2008, it's possible for SQL Monitor to show incorrect values for the Disk Space Metric. The source of this issue isn't actually from SQL Monitor itself, but rather from the monitored instance with Windows Server 2008 OS. 

Microsoft has released a hotfix that once applied should resolve this issue with SQL Monitor: https://support.microsoft.com/en-us/help/961435/some-base-values-are-not-updated-when-you-use-the-win32-perfrawdata-pe