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Here's apple's official explanation:

How to erase a cd-rw

Good luck and enjoy!

BTW, Mac OS X only allows single session use of cd-rws, so you will need to use another program (Roxio Toast, e.g.) to burn multiple session cd-rw's
 
Thanks, it gave me an error message:

Optical Erase Failed
with the error
Whole disk required, slice passed
 
Thanks for your help i found out what the problem was...i just needed to select the drive not the actual media and it erased...THANKS!!!
 
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