do a "dmesg |grep usb " or dmesg |more and scroll down and see what device is assigned.
if u dont have a scsi or sata disk most of the time the usb will be /dev/sda1, i hope ue usb will be vfat and u have a /mnt/usb dir. with all these im mind i will edit the fstab as
/dev/sda1 /mnt/usb vfat noauto,rw,umask=000 1 0
now plug the usb in and do a mount command (mount /mnt/usb)or click on the relevent icon in "mycomputer"
note: if /dev/sda1 is not the device then /dev/sda2 or /dev/sdb1 may be then edit fstab like that