All you need for JSRF is the Media folder and the SegaJSRF.xbe file. Rename the SegaJSRF.xbe file to default.xbe and make your xiso with just those files, then burn onto your disc. Total size is around 2.5gb for JSRF alone.
just a side thought (aside from do what RDC indicated) but in your dash what Hard Drive does it say you have?