If you backup your iPhone, iPad or other iOS device to your Mac instead of iCloud, then you’ve probably seen a noticeable increase in size used on your start up disk. This is because iOS backups can be multi-gigabyte files that take up considerable space; however, you may not actually need these backups except in emergencies. This can be especially noticeable on a laptop, where disk space is often limited.
you can move backup files to an external disk so that they no longer take up extra space, first you create a backup for your iOS device using iTunes, connect your iPhone or other device to your Mac, click on the device tab in iTunes, and hit the Back Up Now button or go to File > Devices > Back Up then you quit iTunes, Open a Finder window and type Shift + Command + G, Locate the folders inside of the Backup directory, and move these to another drive, then you relaunch iTunes.
When moving the backup back to your start up disk you must move the backups back to your main start up disk so that iTunes can locate them then you quit iTunes, open a Finder window and type Shift + Command + G then On your external drive, locate the folders previously removed from this Backup directory and move them back to this location and finally you open iTunes and begin the restore process. Once the files are back where iTunes they were originally, iTunes should be able to see them and make them available during the restore process.