4th G "corrupt file" message

[jabberwock]

Got a message on iPod Classic 4th G, 20 GB that says "corrupt file". 1) Why? 2) Can I repair myself? 3) If so, what do I need to do/change? 4) Can I upgrade to 40GB while repairing?
Thanks!

[RapidRepair]

Was this message displayed on your PC screen or on the iPod screen? Have you tried to restore the iPod with iTunes? Check the drives smartdata with the iPod diagnostic mode (Google if you are unsure how to use it). If the drive has a lot of bad sectors then its time for a new HD. you can upgrade to a 40GB drive but you will need a thicker back panel to accommodate the larger drive size. We can give you a deal on the drive and panel combo, call in to find out more.

[jabberwock]

Message was on the iPod screen. The reason I haven't tried to restore with iTunes is that I read a lot of posts where doing that made everything even worse. But will attempt and see what happens.
Thanks for your assistance. I'll get back to you about the 40GB thing, if I get anywhere with this.