Please note, I'm commenting as a 3k user.

The theory is that any "standard" USB stick should work. In practice we've had reports here that some have not cooperated at all.

I'm using a bargain basement unbranded 1 gig stick with my 3k, no problem. I've also hooked up a 20 gig hard notebook hard drive using a USB 1.1 external case, and that also works fine (drawing power solely from the 3k down the usb wire). However 1 gig is proving more than adequate so I'm using that day to day.

Any capacity should work; however you should always format the media on the keyboard (even if it comes ready formatted).

Also I would steer clear of any sticks that incorporate security features because the one's I've seen tend to appear on a PC as a pair of devices. I've seen that confuse a PC so I'd not be surprised if the Yamaha gave up completely!

So I'd get a cheap unbranded non-secure, non-flashy, no extras stick and see how it goes. If you intend using the "record to audio" feature then get the biggest capacity you can afford. Most "branded" sticks have the same chips inside anyway!

I'd assume that the s900 has the same issue as the 3k / 1.5k keyboards in that you can only have a maximum of 250 (ish) entries in a single directory of the storage device.