Depending on the model of your Mac, you might need an external USB drive to install Windows (you’re asked for one when you prepare your Mac for Windows if it’s required). Newer Mac models—with OS X 10.11 or later—use the internal drive to temporarily store the items you need to install Windows and don’t require a USB drive. To find out whether you need an external USB drive, see the “Learn more” section in the Apple Support article Install Windows 10 on your Mac with Boot Camp Assistant.
To install Windows on your Mac, do one of the following:
If your Mac doesn’t require an external USB drive: Follow the instructions in Install Windows on your newer Mac using Boot Camp.
If your Mac requires an external USB drive: Follow the instructions in Install Windows on your older Mac using Boot Camp.