Can you counter a spell that was cast using storm copies?

When a spell with Storm resolves, it creates copies of itself on the stack — one copy for each spell cast before it this turn. These copies are not cast; they are simply placed onto the stack by the Storm ability (CR 702.40c). Because they were never cast, they are not "spells" in the sense that most counterspells target.

CR 112.1 defines a spell as a card on the stack, or a copy of a card on the stack. So the copies are spells in general terms. However, the critical issue is that most counterspells (like Counterspell or Cancel) counter "target spell" — and the copies are legal targets. You can counter individual storm copies with a counterspell that simply says "counter target spell."

What you cannot do is counter the storm copies with a spell or ability that specifically counters spells "cast" this way, or use effects like Stifle to stop the storm trigger — well, Stifle can stop the trigger from generating copies, but that's a different matter. The key point: standard counterspells can target and counter individual storm copies one at a time.

Example: Your opponent casts Grapeshot and storm creates 4 copies. You can cast Counterspell targeting one of those copies, countering it. The other 3 copies still resolve. You would need a separate counterspell for each copy you want to stop.

So the correct verdict is: storm copies can be countered by normal counterspells (they are spells on the stack), but they cannot be stopped by effects that specifically require the spell to have been "cast." You also cannot counter all copies at once with a single counterspell unless that spell specifically affects all spells on the stack.