From my archives:
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Open the Terminal
Assuming your new image is called ~/Desktop/Master.cdr (the file is on your desktop and called Master.cdr) type:
cd ~/Desktop
(this changes the current working directory to the desktop)
Then type:
hdiutil makehybrid -iso -joliet -o Master.iso Master.cdr
This will create an ISO/Joliet .iso file.
TIP:
Use a text editor to "prepare" your terminal commands.
Then, copy them and paste them into terminal.
Doing this lets you see and avoid typos before they happen.