Post taken from:  oxdaily.com

 

Note: Moving the iTunes iPhone/iPad/iPod backup folder to another drive requires that external drive to be connected in order to backup or sync future iOS devices properly.

  • From the Mac OS X Finder, hit Command+Shift+G to bring up the “Go To” window, type the following directory in:

~/Library/Application Support/MobileSync/

  • From this folder, copy the “Backup” folder to a new location on the external harddrive, for the sake of this guide we’ll choose a folder called iOSBackup on a drive named “External”
  • Before trashing the origin folder, rename the existing Backup folder to something else for backup reasons, like Backup2
  • Now we need to create a symbolic link between where the original Backup folder was, and the new location on the external drive. Launch Terminal and type the following command, referencing your new backup location:

ln -s /Volumes/External/iOSBackup/ ~/Library/Application\ Support/MobileSync/Backup

(That command needs to be on a single line, formatting may look otherwise.)

Verify Symbolic Links
To double-check that the symbolic link has been created, open ~/Library/Application Support/MobileSync/ in the Finder and look for the Backup folder, it should now have an arrow on the corner representing the symbolic link (think Aliases in terms of Mac OS X Finder), like the picture below:

link-itunes-backup-to-external-drive

Prevent Automatic Syncing of iOS Hardware
Next you’ll want to disable automatic syncing of your iOS devices, because there are probably times when you’ll be plugging iOS hardware into your Mac without the external drive connected. Doing this is easy, just open iTunes > iTunes Preferences > click on “Devices” and then select the checkbox next to “Prevent iPods, iPhones, iPads from syncing automatically”

prevent-itunes-from-auto-sync

Test a Backup & Remove Backup2
Finally, before removing the “Backup2? folder and saving the disk space, you’ll want to complete a backup and sync of your iOS device to verify that everything is working as intended. There shouldn’t be any problems, but if there are, retrace the steps to make sure everything was done correctly. If the backup is fine, go ahead and remove the Backup2 folder and you can rely on the link to the external drive when it is connected.

That’s it!

Can I undo this? How do I move the iTunes Backup back to its default location?
Yes of course! If you ever want to undo this, simply remove the “Backup” symbolic link (the one with the little arrow icon) from ~/Library/Application Support/MobileSync/ by deleting it, and then moving your iOSBackup directory from the External Drive back to its original location. It’s that simple.

Will this work for all iOS devices?
Yes it should work fine with any iOS device including the iPhoneiPad, and iPod touch. If you use multiple iOS devices, they’re all stored in the Backup folder, which could make this tip even more useful since more backups = more space taken up locally.