itunes could not backup the iphone because there is not enough free space on this computer to save the backup

Ostensibly just what it says. See this post Back Up Problem in iTunes.

tt2

It's really pretty self explanatory... You don't have enough free space left on the hard drive of your computer to hold a backup of the data on your phone.

In that other thread it turned out that disabling anti-virus during the backup resolved the issue.

An alternative approach would be to exclude the %Appdata%\Apple Computer\MobileSync\Backup folder from real-time scanning.

tt2