Boot Camp: Black Screen when installing Windows 7

Hey guys, I have an intel 27' iMac. 4 days ago I removed windows 7 because there was an issue with the system (it didn't let me run Maya), so I attempted to install windows 7 again, but the problem was that my internal disk drive doesn't work. (Tried multiple ways of fixing it) 😟 Then I bought an external disk drive, hoping that this would fix everything, I was able to partition my windows 7, but when the mac restarts for the first time I'm introduced by a black screen with a small blinking dash top-left of the screen. So I found a fix for it, I got a USB renamed it to MBRFAT or MRBFAT don't remember exactly (I'm on a different computer) and did everything the internet told me to do. To my surprise, it made the blinking dash disappear, but I was introduced with yet another error. Bootmgr is missing, these 4 days were filled with complete bad luck. Please help me out here. 😕

Too long; didn't read version: When the iMac restarts for the first time, a black screen appears with a Bootmgr error, telling me to press any key, although I keep pressing it on my USB mac keyboard nothing happens.