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[xboxmods2977]

More info is needed about your xbox for a definitive answer but...

The xbox first attempts to boot from DVD by default unless disabled in bios. If no disc is present, or if the present disc is unreadable, it will try the HD next.

So, this means that if you are attempting to boot from a CD/DVD and it appears that the xbox is overlooking the disc in the tray, it isn't. The disc simply can't be read.

Check the disc and/or DVD Drive.

EDIT:
How is your xbox modded.
What is the error number you get?