A storage device includes a main storage including a plurality of nonvolatile memory devices, the main storage device configured to store data; and a storage controller configured to control the main storage. The storage controller is configured to, divide a plurality of memory blocks of the plurality of nonvolatile memory devices into a plurality of banks, assign each one of the plurality of banks into one of a) a plurality of sets and b) one free bank, each of the plurality of sets including at least one bank, perform data migration operation to transfer the data among the sets by using the one free bank in response to an input/output (I/O) request from an external host device while securing a specific I/O execution time, and control the data migration operation such that the I/O request is independent of the data migration operation.