A non-volatile memory device may include a semiconductor substrate including an active region at a surface thereof, a first memory cell string on the active region, and a second memory cell string on the active region. The first memory cell string may include a first plurality of word lines crossing the active region between a first ground select line and a first string select line, and about a same first spacing may be provided between adjacent ones of the first plurality of word lines. The second memory cell string may include a second plurality of word lines crossing the active region between a second ground select line and a second string select line, and about the same first spacing may be provided between adjacent ones of the second plurality of word lines. Moreover, the first ground select line may be between the second ground select line and the first plurality of word lines, and the second ground select line may be between the first ground select line and the second plurality of word lines. Moreover, portions of the active region between the first and second ground select lines may be free of word lines, and a second spacing between the first and second ground select lines may be at least about 3 times greater than the first spacing. Related methods are also discussed.