If you ever copy and paste a project directory be sure to change the seam-gen.properties file.
I got a new MacBook Pro. I copied my Eclipse workspaces over of course.
I made some changes to the entities in a project. I then opened a terminal and ran "seam generate-ui." I got a FileNotFound exception in build.xml line 1154 . Something was still pointing to the C:/ drive.
Inside the root Eclipse project directory there is a seam-gen.properties file. Changing that fixes anything.