What makes you suppose there is a limitation on size or the Brand of hard drive? I think all that matters is the interface and the filesystem you're using.

im using mine with two 2TB external drives via usb hub.. btw, both of these drives are powered.

Can you do more than 128GB flash drives?

definitely. I'm currently using a 500GB WD Hard Drive in a non-WD enclosure.

I've added a 5tb powered external hard drive and a 2tb portable drive.

Both work fine with a standard usb hub

ExFat drives encounter some problems, though.

IIRC the Mi Box can take up to a 2TB drive, unless I'm confusing it with the Roku 3 which has likely been updated since getting rid of mine..?