Macbook air: external hard drive boot

[vjicecool]

Has any of you guys have lion on external hard drive and booted it on MBA
i am having issue where i see not permitted sign.

 

[669532]

check you've done it right:
http://www.makeuseof.com/tag/install-osx-lion-external-drive-test-mac/

 

[vjicecool]

I did it correctly
I was able to boot from the external harddrive on my MBP
now when i am using my new MBA, i am unable to do it
any insight on this one