Hello an extra recommendation.....
When I demo the expansion packs on the PSR S970,S770 and S670....in my demos I do use Audio files that I have linked to the Multipads, and for experience, I do recommend you to use a USB memory over 32 Gigs or 64 for heavy users, actually, that depends on your needs.....if is a light use, even a much smaller capacity would do (8 or 16 Gigs).
BUT, most importantly, buy the USB 3.0, this allows to a faster transmission and also to capture faster when you record audio, each USB 3.0 manufacturers have different speed of writing and reading, and that info is on the package of the USB 3.0.
In some demos I do, I play simultaneously FROM the USB 3.0, a wave(Acappela), sometimes a SMF, and/or even a style, besides all my Registrations, Multipad files, etc....so, you can say that I really push the limits on the USB, and the 3.0 works flawless....while using the 2.0 (when I started doing the demos sometime ago) and I was pushing the USB, either the Wave stop while playing or if I was recording, just stopped, when I made the change, that has never happen with the 3.0
You can distinguish or see the difference (physically), when it is a USB 3.0 you see a BLUE plastic tab inside of the Flat part of the USB, the same tab in the 2.0 is BLACK.
My 2 cents...
Manuel