I am having the same issue, and it's incredibly frustrating. Note my issue is with an old iPod Nano. (yeah yeah... it's my way of disconnecting when I go to the gym.)
So there are two general ways you can try to do this -- Syncing, and Dragging. I'll go through the experience with both.
When Syncing...
In Finder, I have "manually update" checked, but it still threatens to erase everything when I click Sync. No! I don't want this!


Now, let's say I try to drag music tracks over to the iPod.
There are two ways I have found of doing this, and neither works (note, it actually does weirdly work for some tracks, but not others):
1) Dragging the music FROM the music app TO the iPod icon in the sidebar of the Finder
When I do this, it only lets me drag to the lower icon, which says "iPod" (why are there two icons??? no idea). It updates the music THERE, but then I can't find it when I disconnect the iPod.

2) Dragging the music WITHIN the music app to the iPod icon in the sidebar of the Music app
This is similar to the last experience. It looks like I am adding it to the icon (a little black circle appears as if it's updating), but then I don't see it when I disconnect the iPod.


WHY ARE YOU DOING THIS TO ME APPLE. I AM SO **** ANNOYED!!!!!