A data storage device includes a nonvolatile memory device and a controller which controls the nonvolatile memory device. When the data storage device is powered on after a sudden power off (SPO), the controller detects an erased page by scanning, without decoding, a first system data block of a nonvolatile memory device, performs simple decoding for first system data of first system pages before the erased page, and, if the simple decoding is a fail, recovers the first system data for which the simple decoding failed, by reading out second system data from corresponding second system pages of a second system data block as a duplicate block of the first system data block.