I'm having some trouble installing windows 7 to my MacBook pro mid 2009 model. My optical drive doesn't work so I'm trying to install it from a USB. I have a .iso of windows 7 home professional on my USB. I've installed reFIT and when I booted it up and selected windows it says missing os. What's going on here? What am I doing wrong?
You can't. It's not supported on the Mac. Only the MacBook Air supports installing Windows from a USB optical drive and/or USB flash drive. You have to use the internal optical drive.
It's because the machine doesn't see the USB drive with a bootable sector and isn't smart enough to boot from an .iso. Here's a primer on how to make a bootable Win7 USB. Fusion will see it on a Mac, but I've personally never tried to boot from one as I only use Windows in VM's http://www.intowindows.com/how-to-install-windows-7vista-from-usb-drive-detailed-100-working-guide/