I`m a Mac user , so I don`t know if this well help , but Donny`s right , do a re-format and make sure there are no bad files or folders on the USB drive,
Then load ( without a folder ) a midi sound pulled of the net and then plug it into the S900 and see if that song can be played.
If so , then move on to a folder of song`s.
Best of luck , so Cubase AI comes with the S900 , wow , I know it comes with the Motif XS.
So you got 2 disc`s with your KB , one for the drivers / Yamaha song book etc.
And Cubase AI ? wow
Best of luck , please let us know if anything we suggested works.