It keeps removing my previous music I just added

By a long shot, the best  way to use Windows Media Player is without using the full auto sync.  You can do this by manually dragging your target music to the Sync pane on the right, then click on Sync files to device.

This is what I do when using many Sansas and several host PCs. Each one has a different media collection, as well as podcasts.  By going manually, you have control over whether the media will remain on the Sansa.

Bob  :smileyvery-happy: