I was using macos sierra public beta on external usb 3.0 usb. First i try to boot in recovery and reinstall macos sierra. but after installation progress imac boot to prohibitory sign. Then, I downloaded macos sierra from app store and when installed it, imac booted with prohibitory sign. After that i made and bootable usb and try to install it but again prohibitory sign. Then, I boot from internal drive and i used carbon copy cloner and copied internal drive to ssd (el capitan). After that i again try to install macos sierra to ssd with el capitan. but same thing happaned and can't boot with prohibitory sign. Now, I can use external ssd with (corbon copied) os x el capitan but cannot update to sierra. What can i do?