How to Back Up an iPhone to an External Driveby Isobel Phillips
By default, iTunes backs up your iPhone to a folder under your user account on drive C and there is no built-in function to change the backup location from within iTunes. If you use a solid state drive as your primary Windows drive, you may find your iPhone's backup taking up too much expensive storage space. In Windows Vista and later versions of Windows, you can force iTunes to back up to a secondary or external drive by creating a symbolic link from the default backup folder to the new location.
Close the iTunes program. Hold down the "Ctrl," "Alt" and "Del" keys and select "Task Manager." Under the "Processes" tab, close any remaining iTunes processes such as iTunesHelper.exe.
Locate your iTunes backup directory under your user account on drive C. In Windows 8 the default location is C:\Users\<username>\AppData\Roaming\Apple Computer\MobileSync\Backup where "<username>" is the name of your user account.
Move the iTunes backup directory to your external drive. For example, to move the backup to drive F in Windows 8, from the Search charm type "cmd" without quotes into the search box and click "Command Prompt." Type the following into the command window, replacing "<username>" with the name of your user account: Move "C:\Users\<username>\AppData\Roaming\Apple Computer\MobileSync\Backup" "F:\Backup" Press "Enter" to complete the command.
Create a symbolic link to the new location. Type the following in the command window, replacing "<username>" with the name of your user account: mklink /J "C:\Users\<username>\AppData\Roaming\Apple Computer\MobileSync\Backup" "F:\Backup" Press "Enter."