iPhone Backup Cannot be Saved Because of Not Enough Space on Computer

I have been having this same issue, and it took me awhile, but I figured out what was going on (at least in my case):

1. I tried deleting old backups and lock files etc, and none of that worked.

2. According to CMD-i on my HD, I had plenty of space for the backup.

3. Console was giving -36 error

I went to terminal and typed "df -g" and the free space on the main Filesystem ("/dev/disk0s2" in my case) showed a different and significantly smaller amount of free space than cmd-i on my "Macintosh HD".

The difference turns out to be something called "local snapshots" for timemachine. (see here for more information on the concept: Time Machine MobileBackups and here for how to turn it on and off: https://apple.stackexchange.com/questions/85805/how-to-wipe-huge-mobilebackups-f older ). These have nothing to do with your phone, but they do have to do with time machine lying to you about how much disk space is "actually" free. iTunes should be able to access all the space reported by "cmd-i" on your HD, but it seems that it can only access the space reported by "df -g" in the terminal. I am going to file this as a bug in the morning.

I turned off mobile backups at the command line as shown, waited a minute or two for the file deletes to finish, and then turned them back on. At that point both "cmd-i" on my HD and "df -g" reported similar free space. I then restarted iTunes (just to be sure nothing was cached) and my backup proceeded without a hitch.

I think folks who were making files to "fill up space" to make their iTunes backups work were just causing timemachine to fall below the 20% threshold of free disk space and delete the mobilebackups incidentally. I think anyone who has seen that work, this will work for as well.

I hope this is helpful, but I obviously can't make any guarantees, YMMV, etc.

TL;DR:

1. open a terminal

2. type "df -g" in the terminal and compare free space of largest filesystem with "cmd-i" on "Macintosh HD". If they are (nearly) the same, STOP, the below won't work for you.

3. type "sudo tmutil disablelocal" in the terminal and then your password (if/when prompted)

4. wait a couple of minutes (for files to delete in the background)

5. type "sudo tmutil enablelocal" in the terminal and then your password (if prompted)

6. Compare "df -g" and "cmd-i" on "Macintosh HD" again, and see that the free space values are almost the same.

7. (re)Start iTunes, and do your phone backup.

Thanks,

Harry