There are actually a few ways you can successfully install Windows on an external drive. Installing Windows on an external drive would be a great solution to the problem of available space, but as we said, Boot Camp and Windows impose a restriction on installing to an external drive. While Boot Camp Assistant can partition your startup drive for you to make room for Windows, there are bound to be many of you who just don’t have room to spare on your startup drive to install Windows. One of the downsides to Boot Camp and the Windows installer is that it restricts you to only installing Windows on your Mac’s internal drive. It’s a nice capability that lets you select – at boot time – which operating system you wish to use: macOS or Windows.
Boot Camp and Boot Camp Assistant allow you to install Windows on your Mac.