I have facing issue on my laptop have small storage space. So i can not take 128gb size iPhone backup on local drive. Here this step to take iPhone backup on external drive.
I have connected external hard-disc on laptop and make junction created on both side using power-shell.
Open Powers-Shell and go to iTunes default back location.
cmd /c mklink /J “c:\Users\admin.GEMINIGROUPS\Apple\MobileSync\Backup” “F:\iPhone Backup\iTunes backup”
After entering this command, link will create on backup location.
Here below video will helpful