I may be missing something, but it seems like the -l argument for USEDDISKSPACE seems to only apply to volumes mounted by Windows letter system. I've tried using the following arguments for -l:
Volume Label
Path to mounted folder
It may just be the case that check_nt just doesn't handle monitoring volumes mounted as subfolders. Be great if anyone has any insight.
Edit:
To clarify, Windows usually configures drive letter C as the main mounted drive. I am unable to mount other volumes as other drive letters, due to a office policy with GPO that doesn't leave me enough drive letters to mount any more volumes. We can't change the GPO policy or fire the sysadmin that created it. My work around that instead of mounting a new volume to drive letter d,e,f, etc, is that I mount volumes to something like C:\SQLDatabasefiles. Nagios is unable to recognize that the volume mounted on C:\SQLDatabasefiles is actually a volume, and there doesn't seem to be a way to report on disk utilization on this volume.
This has nothing to do with SMB, which is a folder sharing mechanism.
Best Answer
I think the best approach is to check the SMB share directly instead of using the check_nt on a Windows host with the SMB mounted share. SMB shares exist in a logged in user context and not as a system wide service and so the check_nt plugin can not find them.
Take a look at the check_disk_smb_spaces plugin at Nagios Exchange.
http://exchange.nagios.org/directory/Plugins/System-Metrics/File-System/SMB/check_disk_smb_spaces/details