mac geekery

Answer

If you understand how “extra” software copies a DVD for you, the answer actually becomes semi-obvious. Software like Toast first makes a bit-for-bit copy of the disk on your drive then ejects the disk and asks for a blank to burn to. This is why you need several gigs of free space for the process.

Use Disk Utility’s New command (under File) to create a DMG file of your disk. You can then burn that to another disk.