Transfer music to iPod touch with iTunes without disrupting existing music I have a specific album on my Apple Mac in iTunes that needs to be copied to my iPod Touch. I have music on my iPod touch that was from another device (old PC), so I cannot sync the entire music library on iTunes (Apple Mac), as it does not have some of the music I have stored on my iPod, and will erase it.

If I manually select the album I need to transfer from iTunes to iPod, will that then erase everything on my iPod and replace it with the one synced album? Or will it just add the one album I need to my iPod, and leave all the other music on the iPod intact?
You can't add new music to your iPod if you haven't synced your iPod with your iTunes before and yes, all your music would be deleted from your iPod and be replaced with the iTunes-library.
But what you can do is, you can use a software to copy all your music etc. to your Mac and than you can sync it with this Apple Mac. I used iMazing(formerly DiskAid) many times and it works great. Yes, it costs something, but it is worth the price and an awesome tool.
So:

Download iMazing or any other tool that allows you to transfer your iPod content to the mac without iTunes.
Sync your iPod with the iTunes which has this specific album you want.
Now all your music will be replaced with the iTunes library. But because you've copied your songs from the iPod to your mac/iTunes library before(Step 1) it will sync all your music back to your iPod.

That's it.