CD-RW support is not ideal. The only way that I have found to re-use a CD-RW is to erase it and start over. Note that I haven't tried adding to an existing CD-RW (it was formatted for Windows, so the OS thought it was read-only).
To erase a CD-RW, use the Disk Utility in /Applications/Utilities. There's an Erase tab on the right. Choose the CD-RW on the left, click on Erase, prepare to wait for a while. I don't know if this is typical, but it took me about a half hour to erase my CD-RW. As I said previously, however, my CD-RW was in Windows format at the time so YMMV.