Why won't iMac boot off external HD?

Hi I need help setting up a computer and when I connect my external hard drive it won't allow me to boot from it when I start it up holding (option). It lets me pick between recovery and the external and built in hard drive?

If the computere you are setting up is a New Mac and the external hard drive you are using is from an older Mac that is why it won't boot off the external. All New Mac's come with Special Builds of the current version of OS X and they will not boot or run off an older version of OS X Even IF that older version is the same Base version, like Mt Lion 10.8.x.
The version of OS X that is installed on newer Mac's have the correct Drive Package for the newer hardware. The external does not have the correct drivers.

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=-=-=-=-
jsd2 is the "jd2" mentioned in the link I gave. Thanks (again), jsd2.
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