When the machine reboots, it will be booting to your external hard drive where you can set up an account and get to your desktop. Choosing this as your install target should begin the installation process and install MacOS Sierra on your external hard drive. You should see your current Macintosh HD and also your external hard drive. Upon starting the Install feature of Recovery, you will be presented with the option of choosing which Disk the installer is going to do its work on. In Disk Utility you will want to make sure it is set to Mac OS Extended (Journaled).

Many drives purchased from third party retailers may come unformatted or formatted to Windows based schemes like NTFS and FAT32. You will want to ensure that your hard drive is in the proper partition scheme and is available to install to. If your secondary hard drive was just purchased, you may need to open Disk Utility prior to starting your installation. You will then choose Reinstall a new copy of MacOS to begin the next step. In order to boot to Recovery, restart holding Command and R. If you are running the newest version of Sierra, you will be able to use the Recovery Boot option to launch your installation of MacOS to install it on your external drive. You are going to be utilizing this hard drive, Disk Utility and MaOS installer from the App Store. The faster the drive the better and a connection speed like Thunderbolt is preferred. In order to create another boot volume for your Mac, you will need a form of external storage such as a hard drive. Here are the steps to create a bootable external volume to start up to on your Mac. How to Make a Bootable External Mac Hard Drive No matter the reason, having an additional bootable option on an external hard drive is definitely a good tool for any Mac user to have. Another is creating a drive for testing your Mac’s hardware. One of the main reasons is backup of your current system. There are quite a few reasons for why you may want to be able to have an additional startup drive for your Mac. Here is a guide on how to create a bootable external Mac hard drive to run macOS from an external hard drive.

How to partition external drive on imac