When the Video app of the Android TV is being used and a USB device containing videos is connected to the TV, the message No items. Videos stored on your USB mass storage will appear here. may appear and the video files of the USB device may not be displayed.
The loading time depends on the amount of content on the USB device. The more files there are, the longer the loading time will be. Allow the TV to recognize all the files first before trying to access the video files again.