I have a GX77 and use Seagate Expansion hard drives (no power cord). I am using 3 5TB drives, giving me 15TB without wires.
I also have a 10TB Seagate drive (with power cord). It works, but it's slow, lags, and it's loud. Oh, and it has a power cord.
Not sure if there is anything larger that 5TB that will work (without a power cord).
-Harry