Everyday is a school day. Learnt today that if you have say Exchange 2010 SP3 in your internet facing site, and SP2 with Update 7 in your none internet facing site, External OWA may not work!
Reason being is that your SP3 servers, need to have the OWA files from SP2 Update 7 in my case in the <drive>:\Program Files\Microsoft\Exchange Server\v14\ClientAccess\Owa folder. So in my case it has subfolders for 14.2.347.0; 18.104.22.168 and 22.214.171.124
Otherwise external OWA to the non internet facing site fails!
This only applies if you the server in the internet facing site is kinda new and not had the upgrades of the previous servers.
Hope this helps!