var diskGeo = new DISK_GEOMETRY();

cf. DISK_GEOMETRY, PARTITION_INFORMATION

   Private Declare Function DeviceIoControl Lib "kernel32" (ByVal hDevice As Integer, ByVal dwIoControlCode As Integer, ByRef lpInBuffer As IntPtr, ByVal nInBufferSize As Integer, ByRef lpOutBuffer As DISK_GEOMETRY, ByVal nOutBufferSize As Integer, ByRef lpBytesReturned As Integer, ByVal lpOverlapped As IntPtr) As Integer

   Public Structure DISK_GEOMETRY

Used in DISK_GEOMETRY.

public struct DISK_GEOMETRY

Public Structure DISK_GEOMETRY