How to Backup an iPhone or iPad to External Storage Plugged into a Mac

Open 2 Finder windows (Command + N) and put them side-by-side. In the left Finder window, press Shift + Command + G on your keyboard and paste "~/Library/Application Support" into the box that appears and press enter. Then select the MobileSync folder so you don't lose it.

In the right Finder window, open your external drive and create a new folder. Next, open Terminal (Applications > Utilities) and 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 drive into the Terminal window, and then drag the MobileSync folder in as well, and then press enter to run the command.

Now when you back up your iPhone or iPad, it'll back up to your external drive instead. Hope this helps! Any issues, just ask me.