To start with, let me give you the tough pill to swallow – Running Windows 7 on External Disk over USB3.0 and using it on a Mac is going to be a rather difficult work. But if you are up for that kind of shenanigans, make sure you have a Windows computer around to get you out of jail. Now that you have been warned, let us get into installing the OS. 

Things you would need:

1. Windows computer or a Virtual Machine with Windows on it.
2. USB3.0 SSD Drive – You could use a normal USB3.0 Hard disk but it is going to be a bit slow, if you can deal with it.
3. Bootcamp Installer (Download Link)
4. WinToUSB Software (Download Link)
5. Windows 10 ISO (Download Link)
6. And obviously the Target machine itself.

Now to installing the ISO on the external Hard drive. For being a complicated process, WinToUSB makes this process a breeze

1. Install WinToUSB on the Windows machine (read “Or Windows VM”) and start the application.

2. On opening the application, you should see this window. Select the Windows 10 ISO and press Next. 

3. Select the External Harddrive in the window and press Next. 

4. Now to select the partition in which Windows is going to be installed. Make sure both the Boot partition and the System partition are the same. If you have the Professional WinToUSB select Legacy option, but if you have the free version you wouldn’t have the option. Press Next. 

5. WinToUSB will now install the OS onto the Harddisk. This could take a while, and is a good time to get a cup of coffee. 

6. Once the installation is complete, remove the external HDD from the Windows machine. Connect it to the Mac or Target machine, and boot into the USB HDD. Turn on the Mac and hold the Option (⌥) or Alt, to bring up the boot option screen. Select the drive labelled Windows (Drive would be coloured Yellow to indicate it is an external HDD). 

7. Follow through Windows setup process, this might take a while.

8. Once Windows starts, install Bootcamp files (You did remember to download the files?).  Run through the setup to install the drivers. 

9. Reboot into USB again and the installation process is Done.