BOOL DeviceIoControl(
  (HANDLE) hDevice,            // handle to device
  IOCTL_DISK_GET_DRIVE_LAYOUT, // dwIoControlCode
  NULL,                        // lpInBuffer
  0,                           // nInBufferSize
  (LPVOID) lpOutBuffer,        // output buffer
  (DWORD) nOutBufferSize,      // size of output buffer
  (LPDWORD) lpBytesReturned,   // number of bytes returned
  (LPOVERLAPPED) lpOverlapped  // OVERLAPPED structure
);
...
lpOutBuffer 
[out] Pointer to a buffer that receives a DRIVE_LAYOUT_INFORMATION structure. 

lpBytesReturned 
[out] Pointer to a variable that receives the size of the data stored in the output buffer, in bytes. 
If the output buffer is too small, the call fails, GetLastError returns ERROR_INSUFFICIENT_BUFFER, and lpBytesReturned is zero.
...