You can try to reinstall the drivers for your USB drives or troubleshoot. Do this by going into your Device Manager (Start>Settings>Control Panel>System...under the Hardware Tab, choose Device Manager and select Troubleshoot). You can also uninstall the drivers under the Drivers Tab.
Reboot your PC without the drives connected. After boot, plug in one drive and let the system detect it and reinstall the drivers. When finished, plug in the other USB drive and follow the same procedures.
Dell
If this doesn't work, tell us what happened and where it failed. It would also help to know your PC and other hardware models, and any recent hardware/software changes to your system.