bootable doesn't

Made a backup clone with super duper on external partition of 1tb verbatim usb...it shows in prefs as a startup disk option. On startup it all goes well like its starting up, then the international NO sign....
permissions were repaired, same result.

any clue?

imac. 2:16 ghz intel 2 duo, 2 gb

USB booting generally works fine from a bus-powered portable drive as well as a powered USB desktop drive. It may be, given that it seems to be booting from the external for a while, but then halts, that one or more of the boot sectors of the partition may have become corrupted during cloning.

Does this Starting from an external USB storage device (Intel-based Macs) help?

When u do the back up with SuperDuper did you move the selector to "All Files' then you can also go to Options and select for it to Boot off the Ext HD after it finishes, which then confirms that the Back is good. All you have to do after that is reselect your Start Up disk to the iMac. Try Firewire 400 it may even be a bit quicker than the USB.

Thanks for your response. Following advice, changed usb port, made a new bootable clone with the confirm boot startup option. It works. !