I am currently working on a very similar script. I am using the wmi classes Win32_DiskDrive, Win32_LogicalDisk, Win32_DiskPartition, Win32_DiskDriveToDiskPartition to map the logical drive to the partition to the diskdrive to the vmdk file to the datastore. It still requires two steps comparing sizes. But it's working ok. Expect the full script on my website very soon.