Android 6.0: SD卡目录创建失败，尽管有权限

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

if (ContextCompat.checkSelfPermission(context, Manifest.permission.WRITE_EXTERNAL_STORAGE)
        != PackageManager.PERMISSION_GRANTED) {
    // 如果没有权限，则动态申请权限
    ActivityCompat.requestPermissions(activity,
            new String[]{Manifest.permission.WRITE_EXTERNAL_STORAGE},
            REQUEST_CODE);
} else {
    // 如果已经有权限，则进行SD卡目录创建等操作
    // ...
}

@Override
public void onRequestPermissionsResult(int requestCode, String[] permissions, int[] grantResults) {
    if (requestCode == REQUEST_CODE) {
        if (grantResults.length > 0 && grantResults[0] == PackageManager.PERMISSION_GRANTED) {
            // 权限申请成功，进行SD卡目录创建等操作
            // ...
        } else {
            // 权限申请被拒绝，需要给出相应的提示或处理逻辑
            // ...
        }
    }
}