Don't know if this helps you, but I had a similar problem when WindowsXP suddenly refused to accept one of my Nanos directly from Rockbox.
It still worked, however, when I switched into Disk mode manually. You do this by shutting off the Ipod (hold PLAY/PAUSE until it shuts off) and then pressing both SELECT and PLAY/PAUSE at the same time. Now connect the Ipod to your computer. It will probably work.
To leave Disk mode, hold SELECT and MENU for a few seconds until the Ipod resets.