The SD Card slot on the Element Fire TV Edition televisions is treated as the primary port for external storage, since it’s the only one that can be used to store apps, however, a USB drive connected to the USB port is just as accessible from within 3rd-party apps. The only catch is that you cannot have a USB Drive connected to each of the two USB ports simultaneously.
The television will mount both an SD card and a USB drive simultaneously, but if you try to connect a second USB drive while one is already connected, regardless of whether you have an SD card plugged in, the operating system will ignore the second USB drive. Both USB ports can be used simultaneously, but just not for external storage. You can, for example, use a USB peripheral, like a game controller or keyboard, with one USB port and have a USB drive connected to the other USB port just fine. You can also connect a USB hub to either USB port, in order to connect additional USB peripherals, but again, only one USB drive will be mounted by the OS at a time, even if you have two connected to a USB hub.