If those podcasts are only on your iPod, when you directly click sync, it will erase those on the iPod and replace with those of your iTunes library.
But here is a solution. All you need is to get all existing podcasts from the iPod onto your iTunes. Simply use this popular iPod to iTunes program called iPod rip
http://www.gutensoft.com/Utilities/jobos…
Many of my friends recommended it to me when I needed to transfer all my songs, videos, playlists, photos, podcasts etc. from my iPod back to my iTunes.
With just a few simple clicks, it will help you transfer all podcasts and other files from iPod back to your iTunes library.
In this case, all files (both new and old) are on your iTunes library that you can syn with whenever you wish to without deleting any files.
Best luck!
For more iPod support information you can also go to http://www.apple.com/support/
Use this iBackup - iPhone/iTouch/iPod to Computer Transfer program to help you get all podcasts from iPod to your computer
http://ipod-tool.com/ipod-software/anios…
It was designed to help copy all music, videos, photos, playlists, podcasts and more from your iPod to your computer and iTunes in the quickest and easiest way. And with it, we don’t need to worry about any data loss. Then you can sync smoothly.
Just free download it to try. It will save you lots of time on this.
More other tips search at http://google.com
You can use an ipod to computer transfer program to backup the old songs to computer iTunes first.I personally use Cucusoft iPhone/iTouch/iPod to Computer Transfer.It helps me to transfer my songs/videos/photos from ipod to my computer iTunes directly and safely.It works well for me,you can give it a try.
Free download it at: http://dvd-video-soft.com/ipodconverter/…
It even has step-by-step guide on the whole process.
Hope it helps.

DVD ripper is a computer program that facilitates copying the content of a DVD to a hard disk drive.

DVD Ripper can rip DVD to AVI MPEG WMV DivX H.264/MPEG-4 AVC MP4 MP3 WMA excellently.

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