关于输出流存储在sd卡上

我要把输出流保存到SD卡上去，这样写对么？为何我运行后在ddms上sd卡路径里找不到deme01.txt？

et1=(EditText) findViewById(R.id.ET1);        

       btn1=(Button) findViewById(R.id.button1);

       
        btn1.setOnClickListener(new OnClickListener() {

@Override
public void onClick(View v) {
// TODO Auto-generated method stub
try{
String string=et1.getText().toString();
byte[] buffer=string.getBytes();
FileOutputStream foS=openFileOutput("deme01.txt", MODE_PRIVATE);
foS.write(buffer);
foS.close();
String path=Environment.getDownloadCacheDirectory().getPath();
File myfile01=new File(path+"/deme01.txt");
FileOutputStream foStr=new FileOutputStream(myfile01);  
foStr.write(buffer);
foStr.close(); 
}
catch (Exception e){
e.printStackTrace();}
}

 

manifest文件如下

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.j.file_01"
    android:versionCode="1"
    android:versionName="1.0" >


    <uses-sdk
        android:minSdkVersion="8"
        android:targetSdkVersion="19" />
    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>
        
    <uses-permission android:name="android.permission.MOUNT_UNMOUNT_FILESYSTEMS"/>


    <application
        android:allowBackup="true"
        android:icon="@drawable/ic_launcher"
        android:label="@string/app_name"
        android:theme="@style/AppTheme" >
        <activity
            android:name="com.j.file_01.MainActivity"
            android:label="@string/app_name" >
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />


                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
    </application>


</manifest>