Q: Why does iTunes now download my entire library which takes 6hrs each time instead of just syncing new content to add to my existing iPod content? Very frustrating

Because it has to be more secure.

"Because it has to be more secure."

Huh??  Sync operations should always work incrementally, i.e., through comparison of the existing content of the iPod with that of your library (which, depending on the method you use to sync, could be based on membership of playlists, selected artists/albums/genres, or checked tracks.

• Songs selected in your library but not on the iPod are copied across.
• Songs that are on the iPod but are no longer selected in your library are removed from the device.
• Everything else - i.e., songs that are both selected in your library and are already on your iPod - should be untouched,

Calderdale - you need to provide some more details: what model/generation of iPod do you have, and what method are you using to sync?  Have you recently installed iTunes on a new computer and moved your library from an older machine?  There's also a possibility that the metadata (properties or "tags") of the songs in your library have been updated such that iTunes no longer recognizes common content on the iPod ... this can happen sometimes if you are using Windows Media Player as well as iTunes, or if the songs in your library are purchased from the iTunes store and you've made changes to Artist/Album information, etc.