Backing up iPhone 4S to an external HD?

[inasterism]

Hi, I have a macbook air (2012) with a partitioned internal hard drive. Because of this, I don't have enough space on the laptop itself to back up my iphone. I've tried using an itunes library on the external hard drive, however when I try to back up my phone, I get the following message:

iTunes could not back up the iPhone _____ because not enough free space is available on this computer.

Removing files and emptying the Trash will free up additional space.

Does anyone know how I can get iTunes to try to back up my iPhone to the external hd instead of my macbook?

 

[tevion5]

Don't think that will work.

Can't you move some files off your Macbook to the external drive, backup to the Macbook, then put the backup on the external and bring the other files back?