How to get path from stream WP 8.1?

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C#

Windows-Phone-8.1

This is my code for access MyFile.txt from sdCard. I want to append text at the end of the file, but I need to use StreamWriter(String, boolean). How to get path for MyFile.txt from Stream stream?

StorageFolder sdCard = (await KnownFolders.RemovableDevices.GetFoldersAsync()).FirstOrDefault();
StorageFolder myFolder = await sdCard.GetFolderAsync("MyFolder");
StorageFile myFile = await myFolder.GetFileAsync("MyFile.txt");
using (Stream stream = await myFile.OpenStreamForWriteAsync())
{
    using (StreamWriter sw = new StreamWriter(stream, true)) //Here is the error because of stream!!!!
    {
        sw.WriteLine("....");
        sw.Close();
    }
}

What I have tried:

I tried to use "D:\MyFolder\MyFile.txt" instead of stream but I don't know if D: will be always the letter for sdCard.