There are two USB ports. Make certain you're using the appropriate one.
From the Owners Manual...
Though the instrument supports the USB 1.1 standard, you can connect and use a USB 2.0 storage device with the instrument. However, note that the transfer speed is that of USB 1.1.
Compatible USB storage devices:
• Up to two USB storage devices, such as a floppy disk drive, hard disk drive, CD-ROM drive, flash memory reader/writer, etc., can be connected to the [USB TO DEVICE] terminal. (If necessary, use a USB hub. The
number of USB storage devices that can be used with musical instruments simultaneously even when a USB hub is used is a maximum of two.) This instrument can recognize up to four drives in one USB storage device.
Other USB devices such as a computer keyboard or mouse cannot be used.
• The instrument does not necessarily support all commercially available USB storage devices. Yamaha cannot guarantee operation of USB storage devices that you
purchase. Before purchasing USB storage devices, please consult your Yamaha dealer, or an authorized Yamaha distributor (see list at end of the Owner’s Manual) for advice, or visit the following web page: http://music.yamaha.com/homekeyboard/