A storage device is provided. The storage device includes a nonvolatile memory device including memory blocks, and a controller configured to perform write, read and erase operations with respect to a selected memory block from among the memory blocks. If the number of error bits is greater than a critical value in a read operation with respect to the selected memory block, the controller is configured to perform a read reclaim of migrating valid data stored in the selected memory block to another memory block through at least two migration operations. In the read reclaim, the controller is configured to adjust an amount of data migrated in one migration operation.