If you need to boot an Asus Zebook ultrabook from an external drive, such as USB hard drive of CD drive, that is not partitioned according to UEFI specifications, you will have to jump through some hoops. You may connect a bootable drive to your notebook, but it won't boot from it by default. Here's what you need to do. These instructions work on my UX31A, but likely are applicable to similar models as well.
1. Connect your external drive to a USB port.
2. Reboot your notebook and while it boots, hold F2 key. If you miss he right moment, it will proceed to boot normally, in which case reboot and try again. But if you press F2 right when the computer starts up, the Setup screen will show up. Unfortunately, if you go to "Boot" tab now, your external device won't show up in the "Boot options" list, if your device is not UEFI-compatible. You need to disable some security settings to enable it.
3. Go to "Security" tab now and disable "Secure Boot Control" option:
4. Press F10 to save changes and exit. When the computer reboots, hold F2 again to return to Setup screen.
5. Now go to "Boot" tab. This time option "Launch CSM" will be accessible (it was not before you disabled "Secure Boot Control"). Enable this option:
6. Press F10 to save changes and exit again. When the computer reboots, hold F2 yet again to return to Setup screen once more.
7. Now go to "Boot" tab, and your external device should be shown in the "Boot options" list. Select it in "Boot Option #1".
8. Press F10 to save and exit one last time. This time do not hold F2, and the computer will boot from your external drive.
9. Once you are done, return your boot options to the default state. Select your local hard drive in "Boot options", then disable "Load CSM", then enable "Secure Boot Control". One more thing: return to "Boot" tab; you will notice that the option "Fast Boot" is disabled; enable it.