Hi MPK I'm also facing the same issue. Searched some websites also but haven't found any clue as to solve this internal storage problem of mibox. Another different forum posted the dude using multiple apps to try move apps to external storage but can't get it to work also.

This is what he said

"Hey guys,

I have Kodi (XBMC) installed on my Mi box enhanced (4K, 2GB ram). It uses about 1GB of the internal SD for userdata, which leaves almost no free memory.

Since I also want to install some games, I've tried to enlarge the internal SD by trying to move data to the external SD. I've used:
* Link2SD: works, but not possible to move data from internal sd to external sd, so useless in this case...
* Mount2SD: not working, shows external SD as not mounted
* Internal 2 External: not working at all
* Foldermount: not working
* manually editting vold.fstab: not working, no sdcard line

Did anybody try to solve this problem with success and how?

Thanks!"
Hopefully someone can help with this issue.