I rarely do full backups of my iPhone because I don’t usually have 64GB free on my drive but in preparation for iOS7 I knew I needed to do it.

Unfortunately there is no way in iTunes to specify an alternative drive to backup. Luckily I found this post stating you can symlink another folder on a remote drive in the place of your regular backup folder. The OS knows no difference:

 # You may need to move or delete your old Backup folder first.
 ln -s /Volumes/drivename/iBackup/ ~/Library/Application\ Support/MobileSync/Backup