Step 1 - Plug your new External SSD into one of your free USB 3.0 ports.
Step 2 - Download free cloning software onto your Mac.
Step 3 - Copy your Macintosh HD (internal hard drive) to your new External SSD. Be sure to select "Backup - all files" as that will ensure that you create a bootable clone. This may take a few hours to complete so be prepared not to use your iMac during this time.
Set Your New External Drive to Be Your Default
Step 1 - Reboot your Mac like you normally would but when you see the Apple icon, hold down the Alt key until it gives you an option to boot from your external drive. Select your new drive and if everything looks good but just faster, pat yourself on the back.
Step 2 - If you are loving your new-old Mac, then you can also set it to boot from this drive as a default. Simply go to System Preferences - Startup Disk and select your new external drive to be the system to start your computer.
