My Mi Box S has 8GB of internal storage (5GB visible).
I attached a 500GB drive and selected "format as internal storage".
It seems to work as I was able to upload much more than 5GB of movies without problems. However, the play store does not allow me to install new applications! It says my internal storage is full.
So, it seems media content goes to the new drive, but applications are still stuck in the original storage? I don't see any option to choose where the applications should be stored.
I don't like this voodoo behavior, no idea where things are...
EDIT: In some of the apps, I can tap the "storage used" option and select "USB Drive" (with 881 GB available), however, it failes with "Not enough storage space" - that makes no sense!
I have the same issue. I read that you should go to aplications and find "google media storage" and delete data there. Did not work for me though...