On your system, what are the volumes without letters etc? If they're things like swap/pagefile, recovery partition etc, then they can probably be omitted (e.g. SELECT ... WHERE mount_point IS NOT NULL OR drive_letter IS NOT NULL).
On my system, the SSD is assigned the letter C, and I have partitioned my HDD into two partitions with the letters F and G.
OK, so F and G are not being shown in the query results (or more correctly, they are there, but the drive letter is showing as NULL?). If so, that seems like a bug in the extension.