Why just ethnic? It can also be religious, ideological, or based on some economic interest. The US revolution was a mix of tax revolt and ideology. The British were the same ethnic group as most of the leaders of the revolution.
Again, I don't consider de-colonization movements to be separatism. A rough criteria would be that you must be physically contiguous to the state that you're trying to separate from.
Also heed my second criteria: a people who perceive themselves to be inferior to the majority group will never, ever develop ambitions of separatism. It usually goes like: "we're better than these other guys but they keep us under their thumbs, hence we want our own country."