Issue:
An error appeared while closing OptiSurface Designer.
The error shows:
      "Failed to write file: HTK_Write_Stream_File returned an error"



Solution:
This error happens when the osd file opened is on a (external) disk.  When OptiSurface Designer is closed, while the disk is disconnected, this error will prompt.

Make sure the disk is not disconnected when saving the osd file after closing OptiSurface Designer.