Splitting itunes library between hard drives

This should help:

How to split a single iTunes library over two or more media locations

The big thing is to tell iTunes to not keep your stuff organized so that it doesn't move the files from where you put them.

You'll probably need to blow away your current iTunes database, change that critical setting, then re-add the existing folders to your new database.

But you won't easily be able to move the files around. They will need to stay where you put then, or you'll have to start all over again.

In other words:

1. Move your files where you want them.
2. Blow away the iTunes database file.
3. Restart iTunes, tweak the "Keep media organized" setting.
4. Re-add your media.
5. If you want to move your media again, you'll have to start over.

This paid app should do what you want: https://itunes.apple.com/app/id528459889?mt=12
I have personally never tried it though.

My personal (free) solution is less than ideal but still works:

Make sure iTunes is closed and Alt+click the iTunes icon in your dock. This popup should appear:



Click on "Create Library" and choose the path to a new folder on your external hard drive.

From now on, every track you will add to iTunes will be added to your external hard drive.

If you want to add tracks to your internal drive, do the same thing but click "Choose Library" instead and choose the path to the iTunes folder on your local drive which should be ~/Music/iTunes/iTunes Media.

Note that you will only see tracks that are in the library you selected, so you will have to manually select the library you want to play tracks from every time you will switch, hence why I say it's less than ideal.