My detached garage has a subpanel fed by 3-wire plus ground from the main panel on the house. It's 8AWG copper conductors plus 10AWG ground breakered at 50 amps. In the distant future I may run new feeder and take it up to 90 amps.
Although the detached garage and subpanel were permitted by the previous owner, there is no separate grounding electrode system. Maybe that's because there is a gate spanning the outside corner of the garage to the outside corner of the house, so they were considered attached? I'm considering retrofitting two 8 ft ground rods, separated 6 feet apart, and connected to each other and back to the subpanel with solid 6 AWG copper.
However, my garage is surrounded by concrete. I'd prefer not to drill through the concrete, at least as Plan A. There is one accessible strip of dirt at the outside corner of the garage in a raised garden bed. I've probed the depths of this bed with stiff wire and it seems to be dirt all the way down; I don't hit any concrete slab or footing. Assuming this dirt is in contact with all the other dirt on the Earth, and not just an isolated volume, is this an acceptable place to drive two ground rods?
Bonus Question: If I run armored 6 AWG from the panel, across the garage, through the wall, and down into the garden bed, how do I make the transition from the inside to outside of the garage without having to splice the grounding electrode conductor? Should I run it as bare 6AWG? With a box and clamp for transitioning to the outside?
Below is a shot of the raised garden bed, with the corner of the garage being the stucco wall on the right. The drip edge on the stucco wall is approximately the floor level of the garage. I would guess it's solid concrete below that.
Edit: A picture of the garage subpanel. I don't see any grounding electrode conductor to suggest that I already have a grounding electrode system.