Hi Tim
Are you sure the USB stick is going all the way in, as some larger or unusual shape sticks can sometimes catch on the case around the USB port. (It looks like it’s fully in but in actual fact is not)
Try other USB devices to see if these are recognised. (Even if they are not designed for the S900 you should at least get an error message)
Make sure you go through all the settings pages, just in case the USB ports have accidentally been turned off.
Try formatting the stick in your computer with FAT32. (You can try FAT but as this limits the size to 2GB partitions, I doubt your S900 will be limited to that)
Make sure there is no other software or other partitions on the USB stick.
Hope this helps

Bill