Android 读取或者写入U盘时，报错：Permission denied_android mnt/meida_rw

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以下所有操作的前提是： android 系统已经root

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最近在将文件拷贝到U盘时总是 写入失败（Permission denied）

程序清单中已经加入了权限

  <uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE"/>
  <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>
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并且动态申请了权限

 private static String[] PERMISSIONS_REQ = {
            Manifest.permission.READ_EXTERNAL_STORAGE,
            Manifest.permission.WRITE_EXTERNAL_STORAGE
    };
    private static final int REQUEST_CODE_PERMISSION = 1;
    
int write_permission = ActivityCompat.checkSelfPermission(activity, Manifest.permission.WRITE_EXTERNAL_STORAGE);
int read_permission = ActivityCompat.checkSelfPermission(activity, Manifest.permission.READ_EXTERNAL_STORAGE);
       
if ( write_permission != PackageManager.PERMISSION_GRANTED ||
     read_permission != PackageMa
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