They would notice pretty quickly. The California GDP is ~2.6 trillion. NY is another 1.4. Together, they're about 20% of the US economy.

And that's before we start thinking about agriculture. Of course, in that scenario, water becomes interesting.

I'm sure there are individual Spanish nationalists that think the same about Catalonia. Central governments, on the other hand, generally behave rationally (read: in their own interests) instead of emotionally.

No, the central government is as petty as it gets. For instance when the Canaries banned bull fighting a long time ago there was no problem, but when the Catalans did it recently the ban was overthrown by Spanish courts. Don't get me wrong, the Catalans did not bann it because it is not part of "their culture" or because it is cruel, they just did it to piss off Madrid :-)

Also now for the referendum, the central government sent four thousand police officers in cruise ships to the port of Barcelona to prevent voting. Does not seem particularly grown up to me neither ...

I have been thinking about this as I listen to NPR discuss Catalonia. Not being in Spain or Spanish, I think, what's the big deal, let them become independent.

Then I think about the secession desire of some in California. (I'm on the west coast of US). Personally I don't care. It doesn't really make historical sense, but whatever. Of course the talk is of the whole west coast seceding, and I live in Oregon. I don't want to secede, so that I have a problem with.

I suspect the main issue is money, but the secondary issue would be fear of a slippery slope of secession.