Hi Jenny,
A .BUP Backup file (contains the user-defined System settings as well as the complete contents of the internal User drive) is model-specific, meaning it only works with the model on which it was created. So an SX700 .BUP file definitely doesn't work with an SX900.
By the way, I suspect that Keith didn't mean a .BUP backup file at all, but rather files (e.g. Styles, Registration Bank files or MIDI files) that he had manually saved (copied) to a USB stick and to the PC.
Best regards,
Chris