A storage device includes a controller and a nonvolatile memory device having memory cells and reference memory cells. The controller accesses first memory cells of the memory cells, reads first reference memory cells, which are associated with the first memory cells, from among the reference memory cells when the access to the first memory cells fails, determines a bad area depending on a read result of the first reference memory cells, and identifies second memory cells, which belong to the bad area, from among the memory cells to bad memory cells.