On computers with SSD system drives any iOS device backups can take up a significant proportion of the available space. iTunes doesn’t provide a mechanism for choosing where this backup data is stored, but it can be relocated onto another locally connected drive and a symbolic link or junction put in place so that it appears to iTunes as if the data hasn’t moved.This set of steps would place the data at D:\MobileSync\Backupbut the path could be adjusted to suit. Close iTunes while you do this.

  1. Make a new folder called MobileSync at the root of the drive you want the backup data to live on, e.g. as D:\MobileSync.
  2. Use Windows Explorer to move the current Backup folder at <SystemDrive>\Users\<User>\AppData\Roaming\Apple Computer\MobileSync\Backup into the folder you just made, e.g. using Ctrl+X to cut from the current location and then Ctrl+V to paste into the new one. If the backup folder is large this may take some time.
  3. To make iTunes look for the data in the new location copy the text below, open a command prompt, right-click to paste in the command, edit the destination drive letter/path if needed, and then press <Enter>.
    MkLink /J "%AppData%\Apple Computer\MobileSync\Backup" "D:\MobileSync\Backup"
    

    If you’ve installed iTunes from the Microsoft Store use this command instead.

    MkLink /J "%UserProfile%\Apple\MobileSync\Backup" "D:\MobileSync\Backup"
    

It is possible to do something similar with OS X, but in that case it is ~/Library/Application Support/MobileSync/Backup that needs to be moved and then redirected. The redirection command in Terminal looks like this:

ln -s "/Volumes/<New Volume>/MobileSync/Backup" "/Users/<User>/Library/Application Support/MobileSync"

Where <New Volume> is the name of the volume you want to store the data on, and <User> is the name of your profile. If there are spaces in the path wrap with quotes or escape with backslash. It is probably easier to type in ln -s then drag and drop the two folders onto the command line in turn.

Note that while this method works for iOS device backups it cannot be used to redirect subfolders of the iTunes Media folder.

https://discussions.apple.com/docs/DOC-7392#relocate

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