How to Back Up Your iPhone or iPad to External Storage Plugged into a Mac
Disclaimers
By choosing to back up your iPhone or iPad to your Mac, you will not be able to back up to iCloud at the same time. You have a choice of either one or the other. However, you can switch between them manually every so often if you'd like to have a backup in both locations.
If you choose to solely backup to your Mac as opposed to iCloud or both interchangeably, Apple or myself are not responsible for your data, or loss of it. You are now the data handler and it is stored locally on your Mac. Loss of data may occur, although unlikely, due to hardware/software incompatibilities, following the steps incorrectly, or storage failure/file corruption.
Introduction
If you'd like to back up your iPhone or iPad to an external storage device plugged into your Mac, unfortunately, there's no easy way to do it as Apple does not provide a native option for this. However, it is possible if you've got the patience.
Symbolic Links
To tell your Mac you want to back up to another location other than the Mac itself, you need to trick it into thinking that the external storage device is your Mac's built-in storage. To accomplish this, you can use something called a symbolic link.
What are Symbolic Links?
A symbolic link is essentially the same as a shortcut or alias, except macOS and any third-party apps still believe that it's in the location of the symbolic link itself, rather than the original location that the symbolic link leads to.
For example, if you create a symbolic link in "Folder 1" that links to "Folder 2", as far as your macOS or any third-party apps are concerned, the files are still in "Folder 1", even though they're technically in "Folder 2".
Creating a Symbolic Link
Open 2 Finder windows (Command + N) and put them side-by-side.
In the left Finder window, navigate to your external storage device and create a new folder.
In the right Finder window, press Shift + Command + G on your keyboard.
Copy and paste "~/Library/Application Support" into the box that appears and press enter.
Find and select the "MobileSync" folder in the right Finder window so you don't lose it.
Open Terminal (Applications > Utilities).
Type "ln -s " (including the space before the hyphen and after the "s"), but don't press enter yet.
Drag the new folder you created on your external storage device from the left Finder window into the Terminal window.
Drag the "MobileSync" folder from the right Finder window into the Terminal window as well.
Press enter to run the command.
How This Particular Symbolic Link Works
The "MobileSync" folder is where iPhone and iPad backups are stored. A symbolic link has now been created in the "MobileSync" folder that links to your external storage. So your Mac will think it's backing up to "MobileSync", but in reality, it will be backing up to your external storage because that's where the symbolic link in the "MobileSync" folder points.
Move Previous Backups to Your External Storage Device (Optional)
You may be able to skip some of the steps in this section if you are continuing on from the previous section at the same time.
Open 2 Finder windows (Command + N) and put them side-by-side.
In the left Finder window, navigate to the folder you created previously on your external storage device.
In the right Finder window, press Shift + Command + G on your keyboard.
Copy and paste "~/Library/Application Support/MobileSync/Backups" into the box that appears.
If this folder does not exist or there is nothing inside the folder, you do not have any previous backups. There is no need to move any files now and you can skip this section of this article.
All of the folders shown in the right Finder window will have names that consist of random letters and numbers. Drag all of the folders shown in the right Finder window into the folder you created previously on your external storage device in the left Finder window.
Your previous backups have now been moved to your external storage device for consistency.
Initiating Your First Backup
Connect your iPhone or iPad to your Mac via a cable.
Open Finder on your Mac.
Click the name of your iPhone or iPad in the Finder sidebar.
In the "Backups" section, tick "Back up all of the data on your iPhone/iPad to this Mac".
In the "Options" section, tick the box next to "Show this iPhone/iPad when on WiFi".
Click "Apply" and then "Sync" at the bottom right of the window to begin the backup.
Conclusion
Now when you back up your iPhone or iPad, it'll back up to your external storage device instead, but your Mac is none the wiser. Hope this helps!