Maybe it will help someone: 100% non scientific, but if you format the USB drive directly from the USB port of the Xiaomi Mi Box (with the 6.0.1 update), it works. I tested with three drives, they all worked: that is, formatting went ok, the device recognizes the drive as adoptable storage. I had been doing it thru a USB hub, and had a lot of issues. You can replug the USB drive thru the hub once it is formatted and Android doesn't mind.
HIH someone, I was going mad with it
I formatted a smaller ext hard drive (<500gb) to act as dedicated space to run all my emulators and it worked flawlessly...until the MiBox went to sleep. After that it never read the information stored on the drive again. I tried this three other times all achieving the same results. I had read a few months ago that the MiBox only reacts well with smaller jump drive style devices. Since then I have rummaged through my belongings and managed to produce an 8gig flash drive and 60% of the time it works 100% of the time.
The only time this 8gig drive doesn't read seems to be after a long sleep. I'll either go into system settings and check to see if it is just waking the drive and reading the comments or if I have to unplug it and plug it right back in. eS file explorer also seems to be forcing it into a weird loop of "/sdcard not detected".
That's my sad little fix. It ain't pretty, but I get by.