A method for operating a data storage device which includes a nonvolatile memory device including a plurality of memory blocks, includes generating a valid page count table including the number of valid pages of each of closed blocks among the plurality of memory blocks in which data are written in all pages thereof and the number of valid pages of at least one open block among the plurality of memory blocks in which data is written in a part of pages thereof; generating a valid page scan table including a scan pointer for scanning the number of valid pages of the open block; and backing up the valid page count table and the valid page scan table in a meta block among the plurality of memory blocks.