Are you polling this Windows host from Orion using SNMP or WMI? My script above is for SNMP nodes.
"How does SolarWinds link up the volume I've created with the SDK to an actual volume on the Windows host?"
For SNMP, this works by matching up the VolumeIndex property with the corresponding SNMP volume index (1.3.6.1.2.1.25.2.3.1.1).