Connect the external hard drive to your computer
Connect your iPhone to your computer using a USB cable
Unlock your iPhone and tap Trust This Computer if prompted
Open the Photos app on Mac or the Photos app/File Explorer on Windows
Import the photos from your iPhone to your computer
Open the external hard drive in Finder on Mac or File Explorer on Windows
Create a new folder on the external hard drive if needed
Copy or move the imported photos from your computer to the external hard drive
Safely eject the external hard drive
Disconnect the iPhone and the external hard drive