I was able to get my ATV to boot to the external drive with OS X the first time, but then I rebooted and got the prohibited symbol.
I brought the drive back to my MBP and change the partition tryp so the MBP would mount it. I then removed the kext files, re-blessed the drive and verified the AppleFile was patched properly. Then I changed the partition type back to ATV.
I am still getting the prohibited message. Any ideas?