From what I've learned, this is just a bug with the Windows OS. When you first connected your iPod, it was given a drive letter. When you boot into RockBox, the devices partitions appears different, and the proper partition is given a new drive letter. However, because RockBox shows you the first partition (required for OFW, not required for RockBox), windows attempts to mount this drive with a letter, but fails because it isn't a recognized format. You can remount in iPod mode, and remove the drive letter to avoid this future forward, or format the entire drive for RockBox, or ignore it altogether.