It could be your charger affecting your usb detection.I don’t have 1 to test this,though.Eairler versions could only read one slot,the bottom slot.Mine reads both slots in wiimc now,but it could have to do with the type of usb i’m using.I have 1 that works and 1 that does not.I also know wiimc has to be running under ios58 for dvd access,i’m not sure about usb.You should get a popup if your not running under ios58.Also make sure your using the original meta.xml file that came with the wiimc download,it contains a line that is needed during loading of wiimc.
If you have folders on your sd,they should show up.The only files that will show,are file types that are used by the mode you are in. like .png .jpg for image mode and .mp3 .wav for the headphones or music mode.