I have a small sub panel in a detached garage. It is grounded to the main panel which feeds it, but not to it's own ground rods.
How far from the sub panel can the ground rods be? I either have to drill through a slab to drive the rods just outside the wall the panel is on, or run the ground wire over a door opening to where there is exposed earth (about a 12' total run).
Actually you may need two ground rods at the unattached structure. Don't go over any doors or have a long run of grounding electrode wire. The wire is supposed to be run in as straight a line as possible. Remember. The ground rods and wire are for lightning protection. I would want that fault to have a very short path to earth.