Currently Being ModeratedOct 3, 2012 5:02 PM (in response to MD122)
10 GB is pretty marginal. The problem is that previous backups were "incremental", recording only changes to the previous backup. Now you are making a full backup of the iPhone 5 (iTunes will keep the last iPhone 4 backup around) so you need enough free disk to do a complete backup rather than incremental.
Try restarting your Mac. That usually frees up a lot of memory (temporarily).