说明：  磁盘文件的安全删除，我们通常所执行的删除操作并不是真正的删除只是清除了文件的fat表内容，文件数据还在磁盘上，可以恢复，所以不安全。
(disk deletion of the security document, we usually enforced by the delete operation does not really delete the file will simply remove the fat table of contents, document data on the disk is, can be restored, therefore unsafe.)