OS X :: Best External Hard Drive Configuration For Macbook?

So a little bit about my situation: I have a macbook pro and recently moved my iTunes library to an external drive since I was running out of space on my computer. Obviously I changed the library folder to the one on the external in preferences ect. The only problem is that I'm not carrying around the external with me all the time, in fact, recently I connected the hard drive to sync the month's worth of new music I got and even after changing the library folder to the external one, only about half the songs from my whole library would play.

It's really a pain to add new music to the hard drive too because it keeps prompting me to replace the files. Long story short, I'm obviously not going about this right. Is anyone in the same situation as me/do you know of a better way to keep track of a library on external? If I didn't explain clearly enough (which i probably didnt) dont hesitate to ask.