1.Open up your startup disk, then open up the folder Applications, in applications open up the folder Utilities, in utilities open up the program Disk Utilities.
2. Once disk utilities is open, in the left side of the window, you will see a list of devices. Under your startup disk for your CDRW disc, click on it,
3. in the middle of the window, a few tabs will appear, one is named Erase. Click on that tab. At the bottom right there will be a button that says Erase, click that and wait a minute or two and the disk will appear on your desktop.