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?