Virtual Directory for file is not working with my AMS 5

Hello, I've read discussions about issues with File.copyTo. But I have still a problem. I am upgrading my server from FMS 3.5 to AMS 5.0. In the conf/Server.xml I changed the option for Virtual Directory for file, to enable it :

<VirtualDirectoryForFile enable="true"></VirtualDirectoryForFile>

In my Application.xml, I  have this :

<ScriptEngine>

     <FileObject override="no">

          <VirtualDirectory>/DataFlash;D:\DataFlash</VirtualDirectory>

     </FileObject>

</ScriptEngine>

Then I tried this example from documentation (http://help.adobe.com/en_US/adobemediaserver/ssaslr/WS5b3ccc516d4fbf351e63e3d11a11afc95e-7eacSSASLR.2.3.html), File.open, I tried this in my .asc file :

this.acceptConnection(client);

var logFile = new File("log.txt");

if(!logFile.exists){

     logFile.open("text", "append");

     logFile.write("something", "somethingElse")

}

At this point, it's working, when I run the scripts, I've got new log.txt file in my Application folder (where AMS is installed).

But if I change the script like this :

var logFile = new File("/DataFlash/log.txt");      

The, I get an error :

Error: File operation write failed.

It seems that my Virtual Directory for file is not working.

I've got something working with a virtual directory for stream, but it's KO for file ! I don't understand why.

In my vhost configuration Application.xml, there is nothing special.

I have also a false return when I try :

myDir = new File("/DataFlash");

trace(myDir.isDirectory);

D:\DataFlash is settled why read and write permissions.

I don't want to use folders in install path (C:\Program...).

Can anyone help me to understand what is wrong with my Virtual Directory for file ?

Thanks