How can I sync new music without deleting existing songs?

Silly question, but are the songs already on the phone? Click on your phone in Device and then wait about 10 seconds for music (if any) to populate window.

I tested this yesterday that if you drag songs to the phone that are already on there, it won't copy. This would only duplicate what you already have. Another thing I noticed is if you drag music over to the phone, it takes about 10-15 seconds for the music to be transferred. When it starts you will see a little round progress icon next to Devices>phone. The transfer is usually really fast when you see that.

And make sure "Manually Manage Music...." is checked in your sync options. I can't remember if it is not checked, whether or not would show up on the side bar in Music app under devices. Keep us posted

Thank you for responding picas! I will try this the next time I add more music to my Mac. Do you know if you have to do a “Sync” after you use this method? I think if after pressing Sync you had the following options: 1) Update music with new music since last Sync. 2) Leave music files in place. 3) Replace entire library using Mac music files. Something like that. With option 1, syncing should take less than a minute to add only new stuff.

One other thing to note, when choosing to sync a selection of artists, there should be a way to add artists to that at next sync without syncing everything. Again, long time for GB of music. Thanks again. I look forward to trying the drag to phone in music app method. Maybe Apple is working on this.

I finally had a chance to test this out and in fact that does work perfectly without having to re-do a Sync. I did check Manually Manage Music. I suppose at some point it wouldn't hurt to do a "complete" Sync. Thanks again picas.

I would click the Apply button after you check the Manually Manage box. Even if it syncs, it shouldn't sync your music library since it now is managed by you. Then try adding music by doing what you have been trying to do. Keep us updated.