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It sounds like some MP3 tracks that were ripped from a CD aren't able to be synced to the iPod nano. Take a look at the article linked below, it has some great tips for resolving issues with problematic files.
Make sure that you can play the songs in iTunes
- Double-click one of the affected songs in iTunes.
- If prompted to do so, enter your iTunes Store name and password, then click OK.
- Select your iPod in the Source pane, then choose Sync iPod from the File menu.
If the songs don't play in iTunes, skip to the "Problematic files" section below. If the songs do play in iTunes:
- Click here to find out which version of iTunes you have, and to download the latest version.
- Make sure that your iPod software is up to date; click here to find out which version you have and how to update.
iTunes won't send some files to the iPod
Make sure that the files are in a format that is supported by iPod.
Specific purchases from the iTunes Store don't play
- Try deauthorizing and reauthorizing your computer. Repeat the deauthorization and reauthorization process for any other iTunes Store accounts for which your computer is authorized.
- Double-click one of your affected iTunes Store files.
- Enter your account name and password in the resulting dialog.
- Click OK.
After you've deauthorized and reauthorized your computer, open iTunes, select your iPod, and choose Sync iPodfrom the File menu. If that doesn't resolve the issue, try restoring your iPod.
After you've restored your iPod, open iTunes, select your iPod, and choose Sync iPod from the File menu. If that doesn't resolve the issue, try recreating your iTunes library.
After you've recreated your iTunes library, open iTunes, select your iPod, and choose Update iPod from the Filemenu.
If none of the above steps resolved your issue, there may be an issue with the music files. If you have a backup of the affected song files, delete the files from your iTunes library and then add them back in from your backup. If you haven't been keeping a backup of your purchased music, you can find out how to do this here.
If the affected files came from an audio CD, try importing them into iTunes again from the disc. After you've replaced the affected files with copies from your backup or CD import, open iTunes, select your iPod, and chooseUpdate iPod from the File menu.
iPod nano and long slide shows
If you're using iPod nano, and you view a slide show for over 8 hours, you may notice that the iPod nano display may indicate that a song is playing when in fact it is not or it immediately skips to the next song repeatedly. To resolve this, just reset iPod nano.
A note about copyright
This software may be used to reproduce materials. It is licensed to you only for reproduction of non-copyrighted materials, materials in which you own the copyright, or materials you are authorized or legally permitted to reproduce. If you are uncertain about your right to copy any material, you should contact your legal advisor.