Without getting into Code specifics, I would put the problem with the detached garage like this:
1. If you pull an EGC to the subpanel, then you treat it like a feeder, and DO NOT drive a ground rod, nor do you bond the neutral at the subpanel.
2. If you leave the EGC to the subpanel out, then, you treat the subpanel like a service. You would drive a ground rod, and bond the neutral at the subpanel.
In example #1 the fault current would travel along the EGC, back to the main panel, through the main bonding jumper, out to the utility transformer, back through the MCB, through the feeder breaker, through the feeder, and trip the branch circuit OCP.
Example #2 would basically be the same except the fault current would travel on the neutral from the subpanel back to the main panel.
electure,
I hope the above makes sense to you. I think if you do some research, you'll find it to be true. Good Luck.
