Installing windows bootcamp external HD rMBP

Hey all,
I'm aware you are able to install Windows via a usb stick through bootcamp, however my question is whether you are able to install it from a USB external hard drive. I've loaded up a Windows 8 .iso on to my WD My Passport and tried to get it going through bootcamp, but every time I try I get "Your bootable USB drive could not be created."

I have made sure the image is not mounted. Any ideas? Does it just not work on an external HD and have to be a flash drive? Thanks.

 

I'm not actually trying to install W8 ON the external drive. I'm trying to install it internally, just install it OFF the external. Exactly like one would do with a USB Flash stick. I just don't have a stick bigger than 4gb convenient.

 

Ok this is beginning to get quite frustrating. No one has any ideas?

I've tried 2 different .iso's, my external HD, and a USB stick now as well. Same problem every time. Any input would be greatly appreciated.