OK, I've heard back and as I stated originally, you should not see the "C" drive
as a separate drive as it is used for caching. He told me that the Surface Data
Eraser should put the drive back to the correct configuration. documentation at
https://docs.microsoft.com/en-us/surface/microsoft-surface-data-eraser the
download itself is at
https://www.microsoft.com/en-us/download/details.aspx?id=46703