What USB drives are you using as the keyboard may not like some of them. There's a list of compatible drives to use if unsure.

Also, have you run some other checks or tests, like can the keyboard read any other files OK such as midi files, styles, multipads, regs etc.... if the SX900 can't read anything at all then it's pointing towards hardware either with the drive or God forbid the keyboard itself... However, if it does load any other types of files, then I think you may have a corrupt install file? Try downloading and extracting it again, put the file in the root of the USB stick, eg, not in any folders etc..

Hope it works out.
All the best.