i found out what those files are and you cannot prevent their creation. HOWEVER, i do feel that apple could have picked a better place for them to be located.
- go into your applications folder and locate Final Cut Pro.app
- right click on the application and "get info"...if the application is more than a couple hundred MB, then you need to clean it out.
- right click the application again and "show package contents"
- inside the MacOS directory, there are more directories with names of projects you are working on. There are also more inside a "Render Files" directory in the same location.
I found these after i switched to high definition and i noticed my system boot/applications volume was running low on disc space...about 120GB lower than usual haha. it turns out these are graphic and transition render files. every time you render an image or text graphic or final cut transition, you get a file in this location. I wonder if final cut's built in media cleaner has something to help clear these files.