Can't manually sync iPhone with iTunes?

I have an iPhone 3G, and I'm trying to manually sync it to my iTunes. It worked on one of my computers, but I'm unable to manually drag music or movie files when it's connected to another computer. Does it only work with one computer? My brother uses an iPod Classic and he is able to manually sync music from any computer. Is the iPhone incapable of this feature?

Obviously if you were able to do it before, the iPhone couldn't be incapable of doing it. Have you tried checking the "manual management" of music and video box in iTunes?

" I'm unable to manually drag music or movie files when it's connected to another computer."

Why not? What happens when you try? That's a big clue as to your problem and should've been included in your question.