You can try to format the USB stick directly on the SX700:
Press "[Menu] > Utility > Storage" and check if the connected USB drive is displayed here.
If yes, select it and execute the Format option. (Make sure that you don't accidentally format the User Drive!)
If none of your USB sticks is displayed at "Utility > Storage", the USB-to-Device socket is probably defective (or even an internal component of the USB assembly).
There is an internal Test Program that you can use to check that the USB ports are working properly. All you have to do is start the Test mode, select/execute the relevant Test item, then connect the two USB sockets (to-Host and to-Device) with a USB type AB cable. If "NG" is shown on the LCD, something is wrong.
The connected USB stick can also be tested (with another Test item).
If you want to carry out these tests yourself, you are welcome to send me a PM (Personal Message), then I will send you detailed information about the mentioned Test program.
Best regards,
Chris