This is for Nano, but also works for any Ipod, I think. And it's for Windows
1) Plug Ipod into computer
2) Click on your Ipod icon
3) Go to Options, then click Manually manage music and videos.
4) Go to Start, if your using Windows.
5) Search for Folder Options
6) Click on it, then click View
7) Go down and find Hidden Files and Folders, check it so it's Show Hidden Files, folders, or Drives.
8) Then click Apply, then Ok.
9) Go to your personal folder
10) On the Computer Tab find your Ipod.
11) Click on it, then you'll see iPod_Control
12) Click on it.
13) Then click Device
14) Then on the top, there should be _locked, rename it to _unlocked.
15) It should work now.
I know this post is old, but some people might find this page.