two itunes libraries - one on external hard disk I have a macbook pro as my everyday machine, but since my music collection was getting big and my hard disk is small, I decided to utilize an external hard disk that I had and was not used for anything. On the external HDD I created a new iTunes Library and copied all my music there. Some small amount of it I left on my MBP so I could listen on the go. When I was home and I wanted to refresh the MBP's music, I'd connect the external HDD and if I downloaded any new music in my iTunes Library I'd copied them to the external HDD too.
This way the external HDD contains the most current contents of my music collection.
It was ok, until I recently decided to buy an iPhone. 

I synced the iPhone to my MBP's iTunes Library and now trying to copy some new music from the external HDD iTunes Library I'm being told that in order to do that I have to delete everything that's on my iPhone.

Is there any different option?
iPhone can be synced to only one iTunes library at any given moment. If you want to add songs to the iPhone, you must add them to your MBP's iTunes library and then sync to the iPhone.