1. Launch the macOS Terminal application. This can be found using Finder, under Applications / Utilities / Terminal. Alternatively, it can be launched using Spotlight by pressing ⌘ Cmd + and typing Terminal before pressing ⏎ Enter.



2. Instruct the Terminal to change the to the default iTunes backup location, by entering cd ~/Library/Application Support/MobileSync and then pressing ⏎ Enter.
3. Reveal the current backup folder in Finder by entering open . and then pressing ⏎ Enter.
4. The Finder window that opens will show a folder named “Backup”. This contains any iTunes backups which are already present on the computer. In order to change the backup folder’s location, this folder must be renamed, moved or deleted. As an example, one could rename it to “Backup (Old)”.
5. Once the “Backup” folder has been renamed, moved or deleted, a symbolic link can be created to the desired backup location. Use the following command ln -s [desired-new-backup-path] ~/Library/Application\ Support/MobileSync/Backup. Once this command has been entered, press ⏎ Enter and the change will be complete.
6. After restarting the Mac, iTunes will store its backups in the new location.

Fix Terminal “Operation not permitted”

1. Pull down the  Apple menu and choose ‘System Preferences’
2. Choose “Security & Privacy” control panel
3. Now select the “Privacy” tab, then from the left-side menu select “Full Disk Access”
4. Click the lock icon in the lower left corner of the preference panel and authenticate with an admin level login
5. Now click the [+] plus button to add an application with full disk access
6. Navigate to the /Applications/Utilities/ folder and choose “Terminal” to grant Terminal with Full Disk Access privileges
7. Relaunch Terminal, the “Operation not permitted” error messages will be gone