When you do each burn, starting with first of course, set the options on the software so that you don't finalize the disk. Then you can continue burn new data with each new burn being a new session. When you run out of space, finalize the disk.
Maybe I am wrong since I have only used Toast but I don't think OSX built-in burn software supports multi-session. Maybe I am wrong but I know a few years ago it didn't and I still think that holds true.
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