If you want to preserve your photos, it is a great idea to make certain you have a backup system in place. As obvious as that sounds, there are still more than a few out there who live life on the edge, and have yet to be bothered with backing up their work. For those of you I offer this inexpensive, and mostly automated, Apple specific backup option you can put in place today.
To start off, I would not necessarily recommend defaulting to the most economical route for backing up your precious one of a kind images. While I have personally used this system for several years, everyone puts a unique value on the worth of the photographs (and video) they create. Only you alone can put a price on that exact worth. If you are for example running a business, and part of staying a float is based on the survival of your clients wedding images, then please, I would urge you to skip this, and have a look at the plethora of more appropriate backup solutions that exist on the consumer market, this is a great solution comes to mind. Ok, if you made it past that last sentence, and currently run wild, unafraid, and without a backup solution in place, and also, happen to own a Mac, then lets have a look at utilizing the built in Time Machine application that shipped with your macOS.
I elect to utilize portable hard drives from G-Technology, for the storage of my images. Any number of reputable storage hardware providers will do though. Most Apple computers these days ship with fast internal solid state drives (SSDs), and while they have many advantages over the slower spinning platters of a more spacious traditional hard drive, they lack those larger storage capacities. The tradeoff is increased performance and reliability. So while you could technically get along with one external drive I think a more reasonable example for anyone who takes even a decent amount of images, is to include like I do, two hard drives in the backup solution. One for working off of and storing your files, and the second for backing up that drive. Which brings us to the Process:
On your Mac, open system preferences, and select the Time Machine icon. This Panel below will appear.
Select the "Options…" button in the bottom right corner of the Time Machine panel. By default Time Machine is set to back up what is included on the internal drive that shipped with the computer onto an external. We will have to override this by clicking the plus icon, and manually excluding the internal drive from backing up.
Once the plus icon is clicked, the Finder window will appear. At this point, select the internal storage drive, and any other external sources that you may have connected, and want to exclude from the Time Machine backup. In my case only the internal storage drive labeled as "Macintosh HD" was necessary to select.
Now confirm the exclusion of the internal drive, again in my case it is titled "Macintosh HD" and click Save.
Back at the main Time Machine window, click on “Select Disc”, and choose the new unused external drive you have designated for Time Machine backup duty. In my example two of the the same macOS ready brand, model and capacity drives are being used. I have custom labeled them in Finder, designated as A and B drives. I will mirror via T.M. my main example drive labeled "G-Drive 2018 (A) with a second drive labeled G-Drive 2018 (B). This (B) or backup drive will seek out any drives that are not excluded and attempt to back them up when "Back Up Now" is selected and the automated Time Machine backup begins.
Once the initial backup completes, you will be left with two drives that look identical with the same folder, and file structure present on each drive as you simply allowed Time Machine to create the backup for you, with all of the initial images from your main drive now existing on both drives. Now with a repeat backup system set in place, if catastrophe strikes you will thankfully have a separate copy of your images, protecting yourself from a drive failure that could one day occur.