usually my USB storage devices are registered as /dev/sda* , usually sda1.
try mounting them,
mount /dev/sda1 /mnt/usb
for example, and if it claims you need to specify a filesystem type, its probably
mount -t vfat /dev/sda1 /mnt/usb
depending on your system the drive may or may not be sda1, but afaik it should be sda*