Also David,
Preferably, start with an EMPTY & formatted pendrive.
There can only be Audio songs that you have recorded on this PSR keyboard to avoid that message.
Personaly, I have had no trouble with brands or size, just format a pendrive once in your PSR, & keep it for PSR recording ONLY.
Only recordings you have done on this PSR can be already on pendrive.
That's what I have found on the S900 I have.
Apologies if I have encroached on other posts.
Cheers, Alan.