On my PC it happens some times as a conflict with a mapped network drive.
For example, I have a network drive G: which the iPod will mount as sometimes. I'm not sure why it does this, but when it does iTunes doesn't see it and the only way to tell it's done that is by looking in the Disk manager.
On my powerbook it doesn't seem to do that, but I'm not sure why there is that conflict on the PC.