1) you have to call device->removeAllHardwareBuffer if you want to remove the cache. Besides the terrainSceneNode leaks in DirectX see my early posts about texturing and terrain combo problems. Many people will have this problem too I bet.
2)Use OpenGL.
Thank, it is resolved somehow, i use OpenGL and used removeAllHardwareBuffer but it still leak with loadHeightMap but not when i drop the scene node, clear, and re-add the terrain.
PS: Does removeAllHardwareBuffer have any effect on other part of the engine? I hope it doesn't screw others loaded things when called.