Got it! You were right that it has to do with the VolumeDescription property. But even when copied and pasted the value in SNMP from a MIB walker into my script, it still failed. After examining a GUI-created volume (for the 3rd time) I finally noticed an extra space in the VolumeDescription property vs. what was in SNMP. There are two spaces after the volume label and before the word "Serial". So a volume that had the description "D:\ Label:Data Serial Number 38606a01" in SNMP had the description "D:\ Label:Data Serial Number 38606a01" in SolarWinds.
After changing my code to add the extra space, it worked. I'm working on a script that will add a node, with volumes found on the computer using WMI. When I'm done with it, I'll post it on the forum for anyone else who may find it useful.