Cecil, my understanding is that if a qualifying concrete encased electrode is present at the garage or second structure you are required to use it, based on E3608.1, Look at E3608.1.2 and see if the building has a qualifying concrete encased electrode and if it does use it. If it doesn't you can install a couple ground rods. Each building needs to have a grounding electrode based on E3607.3 but it could be any of the electrodes listed in E3608. If the only electrode present at the building is a water pipe you must use it and in addition supplement it with an additional electrode(s).
