Just drag the files to the CD icon. This will copy them. You can drag-n-drop stuff into and off of the CD to your heart's content until you eject the CD or drag it to the trash.
When you drag it to the trash, the trash icon will turn into a Cd burner icon (radiation symbol) and it will ask you if you want to burn the CD, cancel the drag or just eject it without burning.
You can also customize the finder toolbar and put a burn icon in it. Under view->customize toolbar and there you go. When you want to burn the cd, just click the icon and you're all set.
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When you drag it to the trash, the trash icon will turn into a Cd burner icon (radiation symbol) and it will ask you if you want to burn the CD, cancel the drag or just eject it without burning.