In recovery operations performed after non-volatile memory devices (i.e., flash memories and so on) experience abnormal status, when unstable data pages are found, valid data pages are copied to another physical block from the original physical block directly and the original physical block is not utilized any more, in order to prevent from spreading error. Further, in order to accelerate the determination process, only partial data of a page is read and whether the page is a valid page is determined based on statistic, when finding out which page is a valid page.