You won't loose a thing. Just take your iTunes folder in your Music folder, and copy it to your external drive. Then on your external drive, make an alias of the iTunes folder. Move this alias into your Music folder on your Macbook, replacing the actual iTunes folder that is there now (be sure to remove the word "alias" from the folder name. Lastly, fire up iTunes. If your external drive is plugged in, the alias will point iTunes to your external drive, and everything will be working from there. All your meta data will remain intact.