Backup iphone to external drive

Open terminal.

mkdir /Volumes/ExternalFileStorage/iTunesDeviceBackups/

cp ~/Library/Application\ Support/MobileSync/Backup/ Volumes/ExternalFileStorage/iTunesDeviceBackups/

cd ~/Library/Application\ Support/MobileSync/

rm -r Backup/

ln -s /Volumes/ExternalFileStorage/iTunesDeviceBackups/Backup/ ~/Library/Application\ Support/MobileSync/