This error occurs when iTunes tries to send files to your iPod, but a file (or files) is unusable (something is wrong with it).
To figure out which file (or files) is causing the error, update to the latest version of iTunes and then do this:
- Control-click (Mac) or right-click (Windows) on your iPod icon in the iTunes Source pane, then choose Sync Songs from the shortcut menu.
- Watch the display at the top of the iTunes window while your iPod is being updated. The name of the problematic file should appear in the display right before the error appears. Then, remove the bad file from the list of songs to be synced to your iPod.
- Select the iPod in the iTunes Source pane.
- Click the Summary tab.
- Select the "Sync only checked songs and videos" checkbox (please note that this checkbox is only available for an iPod configured to update automatically).
- Select the appropriate item under Library in the sidebar, and find the bad file in the list. (For example, if the file in question was a song, you would select Music in the iTunes Source pane. If it were a video, you would select Movies or TV Shows in the sidebar.)
- Deselect the checkbox next to the file name.
- Repeat steps 1 and 2 to update your iPod. If you have multiple problematic files in your iTunes library, you may need to repeat the other steps to remove them too.
To prevent future issues, you should delete the unusable files from your iTunes library, then restore them from a backup, or reimport them from a CD.
If you are not able to isolate to any single file, or if another iPod works with the same songs, then disconnect any other devices connected to USB or FireWire, and try again. If the issue continues, then then try resetting your iPod. If the issue continues, then restore your iPod.