How to move your iPhone ... PDF
Refer: https://lifehacker.com/back-up-your-iphone-to-an-external-hard-drive-with-a-te-1762962368
Original Location of Files
/Users/mruckman/Library/Application Support/MobileSync/Backup
Location on External Drive
/Volumes/Passport/ios_backup
Using Home Passport External Drive
ln -s /Volumes/Passport/ios_backup /Users/mruckman/Library/Application Support/MobileSync/Backup
Using Work WD Elements External Drive
ln -s /Volumes/Elements/ios_backup /Users/mruckman/Library/Application Support/MobileSync/Backup