do you want to read and write files to an SD card connected to FX3? We recommend to use a device called FX3S which is suited for this application. We have example projects of the same available as well.
http://www.cypress.com/products/ez-usb-fx3s and code examples are available in the link - http://www.cypress.com/documentation/code-examples/usb-superspeed-code-examples?source=search&cat=software_tools
what is the fast speed?