Oops, umount fail

wjones@raspberrypi-ptrain:~$ sudo umount -f /mnt/USBdisk
umount2: Device or resource busy
umount: /mnt/USBdisk: device is busy.
        (In some cases useful info about processes that use
         the device is found by lsof(8) or fuser(1))
umount2: Device or resource busy
wjones@raspberrypi-ptrain:~$ ls /mnt/
ls: cannot access /mnt/USBdisk: Transport endpoint is not connected
hda1  hda2  USBdisk
wjones@raspberrypi-ptrain:~$