Well yeah you have to move the actual song files from the directory, not from iTunes. If you're not sure what I mean, or if they're not all conveniently located in one place like they should be, a good way to do it is make a folder on your desktop, go into iTunes, and click and drag the songs from iTunes to the folder on your desktop. It's going to take a long time because 1500 is a lot of songs. Then move that folder onto your USB, and then cut/paste from USB to new computer. Then open iTunes and go file > import folder and boom headshot.