All questions related to installations, configurations and maintenance of Advanced Host Monitor (including additional tools such as RMA for Windows, RMA Manager, Web Servie, RCC).
According to Microsoft manuals it should work correctly in such case - Windows function GetDiskFreeSpaceEx should check free space for specified target path
We checked our code, its fine.
Than you may setup WMI test using query like select FreeSpace from Win32_Volume where Label='VMach1'
If you want to display MBytes instead of bytes in Reply field, you may mark "Tune up Reply value" option and use expression like [%SuggestedReply% div 1048576] or equivalent expression [%SuggestedReply% div 1024 div 1024]