How can i tear my terrain to little terrains

Hi, i was tried to make an AI system but i couldnt bake the navigation because my terrain is to big. My friend said i have to make my terrain turned into little terrains. But i dont know how to do it. I looked for some asset but i couldnt find something i want. Could you suggest an Asset to me for that?

look this Kostiantyn Dvornik :: Ideas: Unity split terrain script