I am considering giving my brother a Xiaomi Mi Box S for Christmas. He has a few movies on a external harddrive, which can be played with VLC or Kodi.
My question is, can he somehow upload new movies to the harddrive while it is inserted in the Mi Box?
You can if you have some kind of Samba Server on your Box. I don't remember if Mi Box S has it already installed or you need to install it yourself.
The Gigabit LAN CoreELEC Kodi device, described and linked to in this thread:
https://www.reddit.com/r/AndroidTV/comments/k7n7we/playback_from_an_external_ssd_or_hdd_no_computer/
Has an already included SAMBA file server packaged into it. You can Plug n Play USB Hard Drives into it.
You can then Drag n Drop files across a home Network from a PC / Mac on to connected USB storage. Naturally USB3 HDD’s are faster.
The easiest option is to see if your router has a USB port that supports Samba and use that instead, that way you'll be able to read & write to the drive without needing to run any server software.