On your PC, did you install an additional drive? installed a software that creates virtual drives? created a new mapped drive? if so, then (connect your N8x0 to the PC) right click My computer/manage/storage/disk management, on the right hand panel, scroll until you find your drives from the N8x0, double click on each one and assign a different letter from what it used to get. Open My Computer and it should appear, if it doesn't reboot. If it still doesn't make sure to try different USB ports.