Make the USB drive boot WININSTALL recognize the USB drive to where you want to install Windows. Connect to your Mac the two USB drive, one of which is where Windows will install and the other is WININSTALL you created with the Boot Camp. Windows starts in your USB drive installed in the version you prefer (7, 8 or 10) using the Boot Camp Assistant creates a USB drive to install Windows. You can install rEFInd on the hard drive of your Mac or USB drive.
If you want to create the USB will use a Mac computer first have to install a bootloader, rEFInd is an excellent bootloader that supports Mac OS X (El Capitan), Windows and Linux. There are unofficial distributions calls Hackintosh.
The OS X is an operating system very jealous and hardly does boot on any computer that is not a Mac.