A method of operating a data storage device includes programming non-fully programmed memory blocks at a point in time when a reference time elapses from a point in time when each of the memory blocks is physically erased, acquiring a first interval and a second interval, calculating a disturb index based on the first interval and the second interval, selecting a victim block for garbage collection based on the disturb index, and copying valid page data of the victim block into a free block. The first interval is defined by a point in time when each of the memory blocks is physically erased and a point in time when each of the memory blocks is fully programmed. The second interval is an interval during which a fully programmed state is maintained after a point in time when each of the memory blocks is fully programmed.