invalid ISO file created by disk util?
It sounds as if you created an empty disk image. To create an ISO/disk image of your disk you have to select the volume (selecting the drive may work too) in the left pane of "Disk Utility" and then click the icon "New Image" or «File» → «New» → «Disk Image from "XYZ your volume name"»  



For CD/DVD-ISOs make sure to select "DVD/CD master" as Image Format in the upcoming dialogue and no encryption.



Now your CD or DVD will be saved as an ISO-image with the extension .cdr - you can rename it to .iso if needed.