Q: My macbook won't let me back up my iphone 4s as it says 'there's not enough space on this computer'.

Hey there JasonDarlo,

I was thinking that it may be possible that your library is in a different location than what you are expecting, such as on an external drive instead of the internal drive.

Use this part of the following article to see if that may be the case:

Where are my iTunes files located?

http://support.apple.com/kb/HT1391

Discovering and changing the iTunes Media folder location

Note: You would usually only change the iTunes Media folder location to share music between accounts on the same computer. See this article for specific steps on how to accomplish this. Also, see this article if you want to know how to move your music to a new computer.

1. Mac OS X: Click the iTunes menu and choose Preferences.
   Windows: Click the Edit menu and choose Preferences.
2. Click the Advanced pane. iTunes displays the location of your iTunes Media folder.
3. 
4. You can then:
   • Note where your media folder is, such as for backing up your media.
   • Use the Reset button to reset it's location to the default (which is the iTunes folder).
   • Click the Change button to select a folder for a new location. Once you change this location:
     • If you make a new or alternate iTunes library, the new location will be used by that library.
     • Existing files will not be moved unless you choose File > Library > Organize library and choose the option to "Consolidate files."

Thank you for using Apple Support Communities.

Take care,

Sterling