After speaking with ZyXEL, they are happy for me to make public their firmware fix for media not appearing in iTunes when using their NSA210 NAS drive.

1. Click here to download the firmware from MediaFire. It’s a little over 26 MB in size.
2. Unzip the resulting file. You should find a .bin and a .txt file
3. Sign into the NSA210 administration screen
4. Click Maintenance > FW Upgrade to open the following screen…
5. Follow the on-screen instructions to upload the .bin file you downloaded and uncompressed previously
6. Once complete you should find your firmware level to be shown as V3.20(AFD.0)2010Apr06
7. I’d now recommend turning off and on the media and iTunes server and rebuilding the database.

If you are having further problems and are from the UK then you need to contact ZyXEL customer services via their email or call them on 0845 122 0301. If you are outside the UK, then you’ll need to contact your local ZyXEL support team.