My computer will not rewrite on my cd-rw cd!?

I tried to re-burn my cd-rw disc and my computer says that the disc cannot be modified. I'm on mac os x on a g4 powerbook. Please Help! Is their something that I am not doing right?
For My computer will not rewrite on my cd-rw cd!? try this:

• It probably isn't drivers. Can you re-write on other discs or are you having this problem on all discs? If you can never re-write then you probably don't have an RW drive.
  
  Its also possible that you have rewritten the disc so many times that there's no more room... every time you re-write you lose a little bit of space.
  
• The drivers may be corrupted, reinstall them. From http://fixit.in
  
• Did you check to see if your CD drive it compatible with CD-RW disks? Some drives can only write, not rewrite.
  
• When you burn a CD or DVD and you want to add more data latter you can not close the session.
  
  Your burning software should have a Tic Box that says something about Multisession. Make sure it is ticked.
  
  If you close the session you can no longer burn data to that disk.