PC operating system

Welcome to the Apple Support Communities

You are probably referring to Windows, right? You have different options to install Windows on a Mac. The easiest one is in a virtual machine. A virtual machine is "a computer inside your computer", where you can install all the operating systems you want and they won't affect to the MacBook Pro's hard drive. This is the best way if you want to do tasks that doesn't require a lot of performance.

However, if you want to do heavy tasks with Windows, you have to use Boot Camp > http://www.apple.com/support/bootcamp Boot Camp allows you to install Windows in the hard drive, so you can use Windows with all the hardware that your computer has. First, check what Windows version you can use > http://support.apple.com/kb/HT5634 Then, follow these steps to install Windows > http://manuals.info.apple.com/en_US/boot_camp_install-setup_10.8.pdf

The problem you have is that Windows can't be installed on a external drive, so you will have to install Windows on your MacBook's hard drive because Boot Camp only allows you to install Windows on the internal hard disk

You are welcome