Trying to make a Ubuntu 12.10 boot CD.
On my PC - The selected disc image file isn't valid.
On my macbook - (not recognized)
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I had the same problem on Windows, the cause was a problem in the .iso file downloaded. Verify your .iso file has the correct checksum, the list of correct checksums is in UbuntuHashes page.
If you are on a Windows box, you can use winmd5 utility to get your downloaded .iso checksum. If on Linux, use md5sum command line utility.