急急急！！！求帮助

private void savetosd() {
// TODO Auto-generated method stub
String str=ed.getText().toString().trim();
if(Environment.getExternalStorageState().equals
(Environment.MEDIA_MOUNTED))
{
File path=Environment.getExternalStorageDirectory();
File saveFile=new File("/mnt/sdcard/LOST.DIR/mydata.txt");
BufferedWriter write;
try {
write = new BufferedWriter(new FileWriter(saveFile));
write.write(str);
write.newLine();
write.flush();
write.close();
Toast.makeText(this, "保存成功", 1).show();
ed.setText("");
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
else
{
Toast.makeText(MainActivity.this, "sd is usefull", 1).show();
}
}
我把输入的文本信息保存后，怎么样改才能使第二次输入的内容不覆盖第一次的内容？
我这样子每次按保存后，旧的内容都被新的覆盖了，求助！