The link between the drive letter, which lives inside the OS running in your guest, and the virtual disk(s) attached to your guest are not obvious.
Hugo made, some time agao, a great script that makes the link based on SCSI IDs.
See Hugo's Get VMware Disk Usage with Powershell post.
In our soon to be released PowerCLI book, my co-author Arnim made an even more advanced script to do this.
But you will have to wait till the book is published :smileycool: