Apple doesn’t officially support installing Windows via Bootcamp on an external device. Therefore you will have to go a different route, but it can be done. The following methods only work if you have another external drive!
There are many guides online, I used this article.
With this guide however, I know that you can’t install it on a partitioned drive since WinToUsb needs to reformat the whole drive, not a partition.
The main steps would be:
- Download the windows support software with the Bootcamp app (open it, hit “action” and then download) and install them onto a pen drive (this includes the drivers, etc. for your mouse, keyboard,… to work under Windows)
- Format your external drive as GUID-partition map and MS-DOS(FAT)
- Install a virtual machine running Windows (e.g. using virtual box), install WinToUsb there and install Windows to the external drive there
- Boot into the Windows installer and finally install the drivers.
You could also follow this guide, however, as it needs to be in Master Boot Record, you won’t be able to use it on your current external drive either.
The main steps from that guide are:
- Prepare your drive/partition (format it as Master Boot Record and MS-DOS(FAT)
- Download the Windows support software with the Bootcamp app (open it, hit “action” and then download) and install them onto a pen drive (this includes the drivers, etc. for your mouse, keyboard,… to work under Windows)
- Use virtual box to install the Windows on the external drive
- Reboot your machine to windows and install the drivers.