As others have said, after turning off power to sub-panel AT MAIN PANEL, run the bare copper ground to the bus on the left of the panel and attach all grounds to this bus. Also make sure there is no bond strap between neutral and ground bus (the big twisted bare wires?).
This is because if you lose the neutral between the two panels for any reason and the ground is attached or bonded to neutral bus, the ground wires back to the assorted circuits could then become hot. This only applies to sub-panels, not to main panels where the ground and the service connect are a short distance (hopefully) from the panel.
Also, you said the wire is 12-2. Either you meant 12-2 w/ ground, 12-3 or you have no ground with your new circuit. Plus remember the red wire is hot along with the black for your combined incoming 240 VAC.