Hello Folks,
I was receiving this error randomly on my macbook 13, OSX 10.6.8 and with various versions of itunes. I'm pretty sure the problem was an excessive number of file entries (Inode exhaustion) in the directory where Itunes stores it's iphone backups:
~/Library/Application Support/MobileSync/Backup/
A typical backup consists of well over 3000 individual file entries.. I had 8-9 different top level backup directories, each with several sub levels.. Bottom line is that I had tens of thousands of individual files in the /Backup/ directory. I believe this was exceeding the available Inodes (data structures that the OSX (Unix) filesystem Uses to hold information about an individual file. When the disk is initially formatted and structured, there is a set ammount of these Inodes allocated). When the available inodes are all used up, Unix sees this as the disk being full (even though there may be hundreds of Gigs available) Since the filesystem has no remaining Inode's to store the relevant information of a new file entry it errors out claiming disk is full. Essentially there are "too many individual files" - A good way of understanding it would be: a 1 GB HDD would hold a single 1 GB file, however it would be unable to hold say a million one byte files.
Anyways, enough of the Unix filesystem theory.. The way I was able to correct my "Itunes could not backup iphone because not enough free space error" was to simply go into:
~/Library/Application Support/MobileSync/Backup/
and delete all of the backup directories in it. From the shell/Terminal I did the following
Open terminal
$ cd ~/Library/Application Support/MobileSync/Backup/
$ tar cvfz all_backups.tar.gz ./* ( this will make a tar archive of the existing backups, may take a while)
$ mv all_backups.tar.gz /some/backup/volume (move your tar archive to some safe place)
$ rm -rf ./* ( remove all existing backup files & directories in the current dir)
once this completes, youe should have an empty: ~/Library/Application Support/MobileSync/Backup/
At this point it may not hurt to go fire up disk utility and repair permissions on the drive that holds your iphone backups.
Fire up itunes, and click on backup. Hopefully all will be well for you.
-todd