Diagnosing fault on ungrounded circuit.

So here's the story:
When I moved in, my garage had some non-so-great 8' fluorescent lights hardwired.  I wanted to replace them with hanging plug-in bench lights.  I took off the old ones and wired an outlet in their place.  (NOTE: These had standard 3-conductor romex, so I thought things were good.)  This gave me a spot for the lights, plus a convenient place to plug in tools.

I thought all was well until I walked into the shop barefoot and touched a screw on the light switch.  Got a little zing off of that.  Also, any tools plugged into the outlets could give a shock.

I grabbed the outlet tester and the magical lights said what I'm sure we all expect right now... no ground.

So I went to have a look at the switch.  Yea, there were three conductors going out, but for some reason the previous owner had run only two multistrand wires in.

"Well, I can sure fix that." said I, and proceeded to run a ground from the switch box to the breaker panel in the closet behind.

And now when I flip the switch... the breaker pops. :/-

I double-checked my own wiring, but that all looks fine.

TL;DR:
I'm thinking the previous owner knew there was a problem in the circuit and decided to leave off the ground to try and hide it.  Not... cool...

So, how does one go about diagnosing where the fault might be?
(There's a floor above and drywall on the ceiling.  Otherwise I'd just run a new cable and be done with it.)

Not an electrician, nor have I stayed at a holiday inn...  Hopefully someone will be by to answer your question.

This sounds like a detached structure.  Does it have its own panel in the garage?  Does that panel (electrical box) have a grounding rod wired to it?  Has the connector at the top of the rod been knock off/loose by a line trimmer cutting the grass at the foundation of the garage?  Is the ground bus in the panel actually hooked up properly?  That ground loop has nowhere to go.  The panel bus and the ground rod would be easy quick checks, and a common failure point for ground.

Your circuit may have reversed "hot" and "neutral" wires at the breaker. I have seen that with amateur wiring jobs.

I have also seen switches that interrupted the neutral instead of the hot. Polarity is okay, but the circuit is always live (sometimes at reduced voltage) even when switched off.

A multimeter is the best tester. If you get voltage from the white (neutral) wire to the earth ground, you know somebody reversed polarity on you. (However, note that in some jurisdictictions it is proper for the wire from the switch to the junction box (supply) to be deliberately reversed; this tells the electrician which wires go to the switch).

Fixed it.
The main culprit I found was, ironically, hidden in my original post above.

You remember that multi-strand wire?  Well, after grounding the box and popping the breaker I took it back apart and noticed a black spot on one of the switches -- pictured below.
(Generally not the preferred method of diagnosis, but hey, we take the clues where we can get em.)

Apparently the old, fat, 1960's switch put the screw terminal very close to the side of the steel box.  This allowed a couple strands of the wire to reach out and tickle the side of the box.

I ran some proper romex with ground and installed a smaller switch.  Much better!

Unfortunately the ceiling boxes still have a faulty ground for some reason, but as these are up high and less likely to zing somebody, I'll call that good enough for now.