I found some information about encountering issues when trying to restore your iPod. These are for people getting a "An unknown error has occurred" error message, but the recommendations should still apply even if you're not getting that error:
To troubleshoot this issue, try the following solutions in this order:
- Disconnect and reconnect your iPod to iTunes, and then attempt to restore or update the iPod again.
- Restart your computer and then attempt to restore or update the iPod again.
- Try connecting your iPod to a different USB port, and then attempt to restore or update the iPod again. Connecting to a USB 2.0 port is recommended. For Windows computers, refer to Identifying USB 2.0 ports on your computer for instructions.
- Remove any USB devices not manufactured by Apple that you have connected to your computer (it is not necessary to remove your keyboard or mouse). Then, attempt to restore or update the iPod again.
- Update to the latest version of iTunes, and then attempt to restore or update the iPod again.
- Try to restore or update the iPod again using a different user account for your computer. For example, create a new test user account on the computer with admin rights, and with iTunes installed, try to restore/update the iPod. For instructions on how to do this, follow steps under the heading "Restore using a new user account" in this article.
- Try disabling Disk Use on your iPod and attempt to restore or update the iPod again. If that doesn't work, try enabling Disk Use on your iPod and attempt to restore/update. Refer to Putting iPod into Disk Mode for instructions.
- If using a Windows PC to restore or update your iPod, try to reregister ".dll" files as described under "Solution 1" in iTunes displays -50 error message when syncing iPod on Windows XP
- If using a Windows PC, try to update or reinstall your Windows USB drivers, referring to the section titled "iPod only: Updating, or in some cases reinstalling, the drivers for your computer's USB interface may resolve this issue."