Whenever I turn off the Mi Box, I have to unplug the power and plug it back in, otherwise it never reconnects properly, or never gets an internet connection even though it shows its connected to the router. The 2nd issue is that with a video file on the USB, I have to do the same power cycle as mentioned before in order for VLC to read the USB again and see that there is a new file on the USB, otherwise it never shows the updated contents on the USB drive. These both seem like bugs. Is Xiaomi ever going to release an update. Its been 6 months and there hasnt been any firmware updates....
I don't have the wifi issue, but you may want to try going into your wifi settings on the Mi Box and forget the saved wifi. Reboot and then re enter your wifi info and connect again.
As for USB, I don't use that but read a lot of people complain about it. That seems something they will fix when they update to Android N.