so that means your flashdrive isn't mounted
in terminal type in "fdisk -l"
as root, that's a dash and little L
do that with usb in computer
it will see usb as sdc1, sdf1, sdb1, etc
then make a mountpoint
where xx is the flashdrive
then mount it
"mount /dev/sdb1 /mnt/sdb1"
now its mounted and should appear under /mnt in file manager
YOU MUST USE THE UNMOUNT COMMAND before you take the usb out!!
if you pull the usb out of computer before unmounting it will damage it