Thanks for the quick replies. Yes Toril you were right on the money. The USB I was using did in fact have a metal case. I swapped it out for another one I had with plastic and it barely even gets warm after two to three hours. It was a Kingston 32GB DataTraveler 50 that I just purchased as it was one recommended on the Yamaha site. It got way to hot for my liking.
on another note....I did find it strange though that if you have the SX900 connected to a computer with the USB cable, that you couldn't transfer data from computer to keyboard other than midi data. I have a home recording studio and thought that the SX900 and computer would communicate better with just one USB cable, and eliminate a flash drive. (Example loading playlist direct from computer.)
Thanks again for replies, much appreciated.