If you have copied a sequence from one project to another in Final Cut Pro, both sequences may end up sharing one or more of the same render files. This will result in not being able to delete the render files for the copied sequence. To change this, open up the sequence whose render files you wish to delete and change the parameter settings of any clips with filters or transitions attached. (For example, go into the Motion tab of a given sequence clip and toggle "Drop Shadow" on and off) This will break the link between the new sequence clip and the old render file.