Mobilesync Backup Direct
( ~ = your user folder. In Finder, press Cmd+Shift+G and paste the path.)
Understanding MobileSync Backup: What It Is and Why It Matters mobilesync backup
C:\Users\[YourUsername]\Apple\MobileSync\Backup ( ~ = your user folder
Use idevicebackup2 from libimobiledevice on any OS: mobilesync backup
This is for advanced users. Ensure the external drive is connected before backing up.