Q: my disk utility is no longer able to erase re-writable CDs why?

Greetings, sharonfromfetcham.

It appears that your MacBook Pro on OS X 10.7.4 won't allow you use Disk Utility to erase your rewritable CD. Since those types of CDs are meant to be reusable, I'll be glad to help.

Use Time Machine to back up or restore your Mac -- Before beginning any troubleshooting, I always recommend having a fresh backup to keep your data safe.

Try the steps below and test after each one is completed. Feel free to skip any steps you've already tried.

1. OS X El Capitan: Burn CDs and DVDs  -- Try the tip, which is listed at the bottom of this support article, to erase the contents of your disk by opening Finder, Control-click (or right-click) the CD/DVD drive in the Finder sidebar, and then select Erase Rewriteable Disc. 

2. Try safe mode if your Mac doesn‘t finish starting up -- Even though your Mac is starting up, this will make sure it's starting up correctly.

3. How to test an issue in another user account on your Mac -- This will help determine if the issue is with your user account or if it's system-wide.

4. About OS X Recovery -- If the issue is happening in the user account you created, you'll need to reinstall your Mac's OS X. Follow the directions in this article to accomplish that.

Thanks for using Apple Support Communities.