Hi there, please, check the type of USB cable you're using, as well as the way you connect the Yamaha keyboard to your Mac. Here's a screenshot of PSR S670 manual with a few details to pay attention to:
AB-type USB cable is the one commonly used for connecting peripheral devices, such as printers, to PCs and Macs.
If your cable / port connections are all correct, I'd suggest to try another USB cable.
In case this doesn't help, please get back here with a more detailed description of your issue, if possible. We'll try troubleshoot this further. Cheers!