How to back up iPhone to external HDD on iMac
iTunes is storing backups by default on local (system) hard drive. That takes heaps of space, so the external HHD would be better choise.
To prevent this from happening, a symbolic link has be to created and point onto a folder in external hdd.
Following command in a terminal will do the trick.
ln -s /Volumes/1TB/Backup ~/Library/Application\ Support/MobileSync/
Just note the following:
1TB = name of external hdd (if it has spaces, then use “1TB”)
Backup = folder on external HDD
MobileSync = does not have folder “Backup” prior running the command
PS: this is general procedure, which might differ based on version of iOS, especially location of backup files on local hdd. Tested on macOs Sierra 10.12.4.