Bootcamp "No bootable device - insert boot disk and press any key"

I want to instal win7 on my iMac, intel i3, but the problem is that when it try to restart after spliting in to partitions by the bootcamp, it gives me this famous message "No bootable device - insert boot disk and press any key" i tried so hard to solve the proB. by intalling with a bootable USB or external HD but i've got the same result... so last week i did change my HD (thinking it will solve the proB) but .... same message "no boot ...bla bla bla"

even i tried to install the win7 that i have in another Mac and guess what !!! it worked

So .... what's the problem ... i want to play VideoGames...

Note that: i've got the same problem from iOS 10.6 to 10.9