I believe that playing samples directly from USB is hard to implement in general (I haven't heard of any keyboard that implemented this feature), and is technically impossible for most keyboards, including PSR-SX900. Let me provide some more technical details. The system architecture of PSR SX900, like most keyboards, is organized like an embedded system, where the UI logic runs on a general purpose processor (most likely a Texas Instrument ARM CPU) and the tone generation (AWM2) is handled by a DSP chip (in this case I'm very sure it is a Yamaha SWP70). The NAND flash that stores the samples is directly connected to the DSP, where the USB to device port is connected to the main CPU. Even with a very fast usb storage device, the bandwidth of the bus connecting the main CPU and DSP is not enough for real time sample playback. Therefore it cannot directly play back samples from the USB drive.
The only keyboard I can think of that has the potential of implementing this feature is the Korg Kronos. It has a very unique design where the whole system runs on an Intel Atom x86 computer and the tone generation is completely software based. This architecture could potentially enable sample playback from external USB drive, but I don't think Korg has implemented this feature.