Booted up OS X. It detected a system update and offered to download and install it. I clicked OK, it did its stuff while I went away and did something else.
Now CD burning with my 3rd party Firewire CD/RW drive works. The process:
- Plug in drive, if necessary.
- Insert blank CD; CD appears on desktop.
- Drag files onto CD.
- Hit black and yellow “burn” button.
- Do something else (e.g. browse web) while CD burns.