For performance reasons, dirty regions are recorded only for level 0 of a texture. Dirty regions are automatically recorded when CubeTexture.LockRectangle is called without NoDirtyUpdate or ReadOnly. For more information, see Device.UpdateTexture.
Cube textures created with Default are not lockable. Cube textures created in video memory are lockable when created with Usage.Dynamic. For more information about usages, see Dynamic.