For my new detached garage I added a 2nd ground rod after the initial inspection. After reading the input from members here I understand why the sub-panel requires two ground rods. My main feeder panel is grounded, 40+ feet from the new garage. In my new garage I had driven the 1st ground rod inside the slab perimeter before it was poured. To add the 2nd ground rod I drilled a 5/8" hole through the slab and drove another 8' rod down to within 6" out of the slab. Added the ground cable from the rods to the ground bar in the sub-panel. Inspector approved.