无法在Nexus 5X And​​roid 6.0上的外部存储中创建文件夹

Question

我在清单中设置了权限：

<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
<uses-permission android:name="android.permission.MOUNT_UNMOUNT_FILESYSTEMS" />

我尝试使用以下代码创建目录：

    // test external storage root
    String myDir = Environment.getExternalStorageDirectory().toString();
    File f = new File(myDir);
    if(f.exists()) {
        Log.d("MyApp", "folder exists: " + f.toString());
    }else {
        Log.d("MyApp", "folder does not exist: " + f.toString());
    }
    // try create folder
    myDir +=  File.separator + "MyFolder";
    f = new File(myDir);
    if(f.exists()) {
        Log.d("MyApp", "folder exists: " + f.toString());
    }else{
        Log.d("MyApp", "folder does not exist: " + f.toString());
        if(f.mkdirs()){
            Log.d("MyApp", "folder created: " + f.toString());
        }else{
            Log.d("MyApp", "cannot create: " + f.toString());
        }

    }

Android 4和5的平板电脑是功能上的，但在Android 6.0的Nexus 5X上看起来像：

D/MyApp: folder exists: /storage/emulated/0
D/MyApp: folder does not exist: /storage/emulated/0/MyFolder
D/MyApp: cannot create: /storage/emulated/0/MyFolder  

在Nexus 5X或Android 6.0中有一些我不知道的甜蜜秘诀吗？