Select the installation disk, partition and mode
On the next screen, use the drop-down labelled ‘Please select the destination disk’. At the top to pick the external drive you want to install Windows to. This will open up a bunch of options below that. You’ll be asked to pick the system and boot partitions. If the external drive isn’t split into multiple partitions, you can easily just pick the available partition for both system and boot.
Finally, you’ll be presented with Installation mode. If you can, pick Legacy mode; that’s the simplest way to go. Only try installing Windows in VHD mode if you have issues installing Legacy. You’ll also be given the option to configure the size of the Windows installation on the drive; just go with the maximum available unless you’re doing something special. Click Next to get the installation started. You’ll see a progress percentage. The time it takes to install will depend on a number of factors, such as the speed of the destination drive. It could be 15 minutes, or it could be a couple of hours. You’ll be prompted when it’s done.