'Back Up' option is greyed out in iTunes?

I want to back up my 120GB iPod classic but on iTunes the 'Back Up' option under the 'Devices' tab is greyed out meaning it can't be clicked. This problem occurs on both computers I use to sync to my iPod, my MacBook Pro and a Dell PC.

Any Ideas? I tried the option detailed here (http://forums.macrumors.com/showthread.php?t=1535741) but it was to no avail.