iPod ipod - "could not unmount disk"

I've got a 30gb iPod video and have a Macbook, recently I thought I'd plug my iPod in and run the "verfify disk" option in disk utility to check that all was well with my iPod. Unfortunatey I get the message "Verify volume failed with error Could not unmount disk". Do I have to have my iPod in hard disk mode to verify the disk? Or is it unnecessary to do this in the first place?

Any help would be much appreciated,

thanks,

Aaron