Try another USB cable, try another USB port on the computer, try another computer, make sure you have checked all the directories... WSD, DFF, WAV, etc. If none of that works, run chkdisk on the drive.
I've not had problems with my Korg USB, but I have a well worn H120 I use for portable playback and sometimes I have to blow the dust out of the port, wiggle the cable etc, for it to connect.