The steps is quite straight forward :
1. In the Finder navigate to Applications -> Utilities -> Terminal and open the Terminal app.
2. Enter this command line by line
cd ~/Library/Application\ Support/MobileSync mv Backup old-itunes-backup mkdir /Volumes/pingo/ios-backup/Backup ln -s /Volumes/pingo/ios-backup/Backup ~/Library/Application\ Support/MobileSync/Backup ls -l mv old-itunes-backup /Volumes/pingo/ios-backup/.
*pingo/ios-backup is your new own directory path.
In case, you have lost connection with the custom path you have set. Here are codes to tell where is the path :
ls -al ~/Library/Application\ Support/MobileSync/Backup
If the backup is not important, you can just replace the broken path by using code below, back to its original path:
rm -rf ~/Library/Application\ Support/MobileSync/Backup
Reference : https://www.iphonefaq.org/archives/975865