The answer for excluding a specific volume can be found here Re: Making Widget out of Alert?
Here is an excerpt from the page:
Excluding particular drives
in alerts you can skip particular disk drives (unable to do that in widgets). Such issue was already discussed at Re: Skip certain volume label or drive letter in the Guest Storage Space Utilization script or Re: Re: How can I exclude a volume from alerting about low disk space? Thwack threads.
If you need to exclude particular disks on particular virtual machines then you can use something like that (this xPath excludes C:\ mount point on "tex-2k8-2LUNS" virtual machine and /boot mount point on "Analytics VM" virtual machine):
then min(/virtualMachine/diskVolume[mountPoint != "C:\"]/freeSpacePercentage)
else if (/virtualMachine[name="Analytics VM"])
then min(/virtualMachine/diskVolume[mountPoint != "/boot"]/freeSpacePercentage)