private String getSDcardPath()
{
    String exts =  Environment.getExternalStorageDirectory().getPath();
    String sdCardPath = null;
    try
    {
        FileReader fr = new FileReader(new File("/proc/mounts"));
        BufferedReader br = new BufferedReader(fr);
        String line;
        while((line = br.readLine())!=null)
        {
            if(line.contains("secure") || line.contains("asec"))
                continue;
            if(line.contains("fat"))
            {
                String[] pars = line.split("\s");
                if(pars.length<2)
                    continue;
                if(pars[1].equals(exts))
                    continue;
                sdCardPath =pars[1];
                break;
            }
        }
        fr.close();
        br.close();
        return sdCardPath;
    }
    catch (Exception e)
    {
    }
    return sdCardPath;
}

if (Build.VERSION.SDK_INT >= 23) {
    if (checkSelfPermission(android.Manifest.permission.WRITE_EXTERNAL_STORAGE)
            == PackageManager.PERMISSION_GRANTED) {
        Toast.makeText(this, "Permission is granted", Toast.LENGTH_LONG).show();

        foldername = "myFolder";
        filename = "test.txt";
        //Сохранение файла на карту SD:
        fullpath = getSDcardPath()
                + "/" + foldername
                + "/" + filename;
        SaveFile(fullpath, "Какой-то текст");

    } else {
        Toast.makeText(this, "Permission is revoked", Toast.LENGTH_LONG).show();
        ActivityCompat.requestPermissions(MainActivity.this, new String[]{Manifest.permission.WRITE_EXTERNAL_STORAGE}, 1);
    }
}

public void SaveFile (String filePath, String FileContent)
        {
            //Создание объекта файла.
            File fhandle = new File(filePath);
            try
            {
                //Если нет директорий в пути, то они будут созданы:
                if (!fhandle.getParentFile().exists())
                    fhandle.getParentFile().mkdirs();
                //Если файл существует, то он будет перезаписан:
                fhandle.createNewFile();
                FileOutputStream fOut = new FileOutputStream(fhandle);
                OutputStreamWriter myOutWriter = new OutputStreamWriter(fOut);
                myOutWriter.write(FileContent);
                myOutWriter.close();
                fOut.close();
            }
            catch (IOException e)
            {
                Toast.makeText(this, e.getMessage().toString(), Toast.LENGTH_LONG).show();
                //e.printStackTrace();
                //textInfo.setText("Path " + filePath + ", " + e.toString());
            }
        }