I have taken the finest toad picture ever seen by humankind...

[Kurt]

As Artie Johnson used to say "very interesting".

[coltiger]

What prevents the snake from attacking/eating the toad? Does it just know by instinct that it is deadly to eat?

[Kurt]

The toad is too big to eat for one thing and I susect toads are not on the menu as far as the snake is concerned. The snakes that eat toads have a tendency to be Bufonid specialists, like hognose snakes

[SludgeMunkey]

I agree with Kurt. I suspect Crotalus sp. being adapted to "see" warm blooded prey may have something to do with it also.