I found a solution ! Eureka !
Apparently Asus have a policy of issuing frequent updates to their Bios'. I updated mine and voila my Internal HDD showed up along with the 6 SATA devices listing. After the update, I set my desired boot sequence and tried several restarts with my Ext HDD connected. No Problemo.
Now, for those of you who have an Asus MB. There is in the TOOLS Menu of the BIOS a utility called EZ Flash which is the preferred method of updating the BIOS. Follow the instructions below
Thanks to all of you who took the time and effort to put down your thoughts on the issue. It was while following your suggestions that finally stumbled upon