Is there a way to divide one terrain into several smaller ones?
I use a program called L3DT. Its a great Land generator in its own right, but it does just what you ask. You simply import your larger height-map and split it into a mosaic map. It will even let you resize on export and it will number each file with a Cartesian suffix to identify it later.
When bringing it into unity, if you chose a tile size of 1000, you simply take the file name and multiply the x and Z values by 1000 and import that into your transform coordinates.
After importing the files you will need to stitch them together. I suggest Stitchscape, it is well worth the money.
Hope this helps
No, unlike the Source or Unreal engines, you cannot cut or sew multiple pieces of terrain, sorry.
If it's just the heightmap you're talking about, export it in raw format and slice it up in Photoshop or whatever, then save the individual pieces back out as raw again and import them into Unity. If you're talking trees, texturing, etc., then it gets more complicated because you have to delve into undocumented APIs. Though technically it can still be done.
I have just released an editor extension that does what you want; it’s called the Terrain Slicing & Neighboring Kit and is available on the asset store for 5$. You can divide your terrain into 4, 16, 64, 256, 1024, or even 4096 smaller terrains. Trees, textures, etc are all replicated. Have a look at this forum post for more information: link text