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The quick answer is Toast has a lot more burning options than Mac OS X built in burn capabilities. With Mac OS X, basically all you get is the ability to copy files from the HD to the CD. With Toast, you can do that, plus lots of other stuff (like video-CD). Refer to the Toast web page for more information.
 
It also supports a ton more drives than Apple's builting DiscBurner software does. I use it for my sony 48x drive i slapped into a firewire case. Apple won't support drive mechanisms that do not come in a firewire case from somebody. Sadly, they do not want you switching out your internal drive, they think it is a bad bad thing to do... haha
 
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