A method of operating a nonvolatile memory device comprises performing a read operation to read data stored in a first memory cell block including first unit groups; detecting a second unit group from among the first unit groups, the second unit group having a number of error bits included in the read data, which is greater than a set number of bits and equal to or smaller than a maximum allowable number of bits which can be corrected through an error checking and correction (ECC) processing; and after the second unit group is detected, performing a copyback operation for moving the data, that are stored in the first memory cell block, to a second memory cell block.