Thanks for this - really helpful as it enabled me to rule out a lack of space as being the cause of the problem in iTunes backup.
While it has enabled me to access a 2TB drive to backup a 64GB iPad 2 it still did not fix the problem
iTunes 'could not back up the iPad (64GB) because there is not enough free space on the computer to save the backup
I've tried everything suggested on the Apple Support HT203271 including the Lockdown Folder (HT203887).
(The error message clearly has nothing to do with a space problem but is something else altogether.)