Unable to find the external s-functions???

Im trying to add external devices via the s-functions for the external mode and this works well as long i add the additional s-functions modules (cpp-files) in the parameter dialog "S-function modules" in S-function-Block. But if i try to add the files via the :

% in rtwmakecfg()
.....
makeInfo.sources = {'sf_my_driver.cpp'};

it throws me this error:

The call to realtime_make_rtw_hook, during the after_make hook generated the following error:
    Failed to save the makefile.

The build process will terminate as a result.

Failed to save the makefile.

Failed to generate makefile.

Failed to instantiate the template for value "# Copyright 2012 The MathWorks, Inc.
# Autogenerated on: 27-May-2014 21:57:37
# Model: SFCN_TESTS
# MyTemplate
# File name: sfcn_tests.mk
# Template: gmake 
# Template Version: 1.0 
# Tool Chain Configuration: Arduinodue
# Tool Chain Configuration Version: 1.0

THIS_MAKEFILE := $(lastword $(MAKEFILE_LIST))
EMPTY :=
SPACE := $(EMPTY) $(EMPTY)

MODEL_NAME := sfcn_tests
HOST_PLATFORM := windows
BUILD_CFG := MW
TARGET_EXT := .elf
TARGET_HEX_EXT := .hex
TARGET_BIN_EXT := .bin
OBJ_EXT := [|||[.obj]MW_XMK_OBJ_EXT|||]
TOOL_CHAIN_CFG := Arduinodue
FORMAT_PATH = $(if $(filter \\

The path 'sf_my_driver.cpp' does not exist

I can't understand why they say it does not exist, because they are definitely there! Can anyone help me? thank you goldeneyes