Ipod Classic - how to stop auto sync

My older iPod classic died. I have another also old iPod classic that I want to transfer all of my music onto (10k+ songs). My MAC sees the iPod and auto syncs but stops after less than 200 songs. I want to try to add music piecemeal, but it keeps autoync all the music. Whenever I try to change the settings, they always revert back to auto sync. HELP!