Having trouble syncing iTunes on new Macbook Pro with library on external drive

Take a look at this article. Maybe it will help you. How to move your iTunes library to a new computer - Apple Support

Setting up preferences only tells iTunes where to put any new media you add, it does not get iTunes to inventory old media. The iTunes Library.itl is the key file you are missing and it is on your old computer (unless it is also on that external drive but you did not tell us that). What you need to do is move the entire iTunes folder from the old computer to the same location on the new computer and then start iTunes. The folder has all the files that give structure to your media files and present what you see when you start iTunes. If you do no have that folder any longer then all you have are a bunch of media files and will have to re-add them to iTunes, build your playlists and ratings from scratch, etc.

How did you choose the library? Choosing the library file requires special start procedures. Are you using those? Realize too that an iTunes library is not just one or two files or a bunch of media, it is all the files in an iTunes folder working together. In other words I am not sure that transferring only the .itl alone will work.

You select a .itl for use by starting iTunes while holding down the option/alt key while starting the application, then selecting the .itl file when it asks you to choose a library. It might well start up this way but you may end up losing all your artwork which is stored in another folder in the iTunes folder, and maybe some other settings.

Only a hundred kB? You either have a very small library or you are using an inactive or orphan version of your library file. It would be helpful to know if all you have on that external really is only media files. You may have an active version of the library file elsewhere.

Unfortunately you set things up to be a split library which makes transferring to a new computer that much more prone to having problems.

The potentially critical thing about that .itl file is that as far as the phone is concerned that is what it recognizes as your library. This may not matter for you but if you make a new library and attach your phone the phone will have to be erased before it will sync with the new library.

If you really do just want to make a new library then simply start iTunes. It will create all the empty files and folders on your internal drive. Go into preferences and set the media file to be the one on your external drive so when you add things they go to that folder. Now go to one of the iTunes menus and there is an "add files" item which you can then navigate to that media folder so it ads the files to iTunes. Alternatively go to the media folder on the external drive and hold down the option key while dragging it to an empty iTunes window and it will add all the files but leave them in their original location.

I was busy editing my earlier reply when you posted. Review for changes.