Installing Windows 7 on a Seagate Backup Pro Slim 2TB

[Siretruck]

I apologize if this isn't the right place for this question, but here's what's going on:
I'm trying to install a copy of Windows 7 on my external to boot windows on a friend's mac, and I'm posting it here because I think it's a hardware issue. I've tried this tutorial
http://www.intowindows.com/how-to-install-windows-7-to-usb-external-hard-drive-must-read/
and I get to the first point in the installation when the computer needs to restart. When I restart the computer, the installation doesn't continue. If I try to start the installation again, it says I need to format the drive and it all starts over. I'm running Windows 8 on this computer and I tried to restart the computer by going to advanced boot options, and restarting and doing the Boot from USB thing, but when it restarts, the computer says "This system doesn't have a USB Boot Option" and goes to that boot up configuration menu. I also tried turning off booting in secure mode as someone suggested on another site and that didn't help.
Does anyone have any suggestions? I'm getting kinda desperate.
Thanks in advance!

 

[spooky2th]

Try tapping the "F12" key when the comp boots and select the external drive.

 

[Siretruck]

Try tapping the "F12" key when the comp boots and select the external drive.

On my computer, it's F2, I think, and it doesn't work unfortunately :/ The USB option doesn't say anything and hitting enter on it doesn't do anything.