Error when trying to back up iPhone to external drive

I created a hard link (mlink / j) from the backup folder on my laptop to a new backup folder on an external drive. (My laptop does not have enough free disk space to backup my phone!). When I try to backup my iPhone I get an error message that a backup cannot be created because the old back up is corrupt and advising me to delete the old backup. There is no old backup on my laptop! I need to do this so I can do a full backup and trade in for a new iPhone. I have uninstalled and reinstalled iTunes but the error still comes up and it will not let me create a backup!