I had the same problem and switched brands of USB flash drives. You'll want to try another brand of USB stick be sure that you have your current USB data backed up somewhere. I don't recall the list of flash drives that are suggested by Yamaha. I use the EagleTec 8 gig drive as do a few other SZ members :
http://usb.brando.com/eagletec-usb-nano-flash-drive_p00892c041d15.html Whatever flash drive you do use I suggest formatting the USB using the keyboards on board formatting utility.
>Function
>>Utility
>>>Scroll to Media Tab
>>>>Format USB Drive
[This message has been edited by Stephenm52 (edited 08-09-2010).]