Here are the steps to be performed on a Mac to create a Windows 7 USB Bootable Installation Drive. If you have access to a different Windows box, try the tool from Microsoft. There are also manual instructions. I don't know of any way of making a windows-bootable drive natively in Mac OS X. I've recently been through this. No matter what I did, I couldn't get my MacBook Pro (can't remember exactly how old it is, but it identifies as 'MacBook Pro 3,1') to boot a Windows install disk off anything except the internal SuperDrive, which unfortunately is broken. I tried more things than I can remember, including external DVD drive, USB key, USB hard drive, Firewire hard drive, EFI bootloaders, etc etc etc.

In the end I managed a hack using a trial version of Parallels Desktop - instructions. If you have a different model Mac to mine, you might find helpful - it's an EFI loader that you can run either from your hard drive or a USB key that should hopefully make the experience of booting into the windows install a bit easier.

How to create a bootable Windows 7 8 USB on a Mac. Click on Go then Utilities. Find the Bootcamp assistant, right click on it and go to Show package contents and then go into the contents folder. Copy the info.plist file to the desktop and make a backup copy of it and open it.