title: android mkdirs in SD card return false.help me, it spends me 3days
body: <p>I want to create a folder in SD card ,and i already add the permission</p>
  
  <pre><code>&lt;user-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" /&gt;
  </code></pre>
  
  <p>in manifest file.below is my code,but mkdirs return false! Can you help me!</p>
  
  <pre><code>File exportDir = new File(
                  Environment.getExternalStorageDirectory().toString(), "happydiarybackup");
          if (!exportDir.exists()) {
              boolean a = exportDir.mkdirs();
              Log.d("mkdir ",exportDir.getAbsolutePath() + " make "+ a);
          }
  </code></pre>