Tried to use a flash drive in one port and it wasn't seen, plugged it into another without a problem. Didn't know if it was simply due to me plugging it in before the system had finished booting up so did an experiment, if I plug my flash drive in and reinsert it into the same port again it still doesn't see it, plug it into another and it does.
So before you reformat your flash drives try it in every USB port first.