Yamaha have a USB drive compatibility ability list on their s970 download list here http://uk.yamaha.com/en/products/musical...970/?mode=model

I haven't spotted a similar list for the pa900 but I'm a Yamaha user so don't really know where to look!

However my experience is that any reputable brand will work, so long as you format the thing using the keyboard and not a pc / mac. The only other gotcha might be that the keyboard wont read a large capacity drive, but I've not had issues up to 16gb, and as mentioned above unless you go mad with audio you'll never need this amount of storage.