NAS Drive questions

Evening all.
I picked up a NAS drive this week on the cheap from someone that didn’t seem to know what it was in one of those used exchange shops in my local town. The model is the HP MV2020, 500GB expandable to 1.2TB according to the box.
I have no need for 500GB to be honest. I have 500GB SSDs in both my rigs as steam libraries.
I have a mate that can take the 500GB that’s in the box if I find I have it spare which is where my questions on the item come from.
From the box it says 500GB upgradeable to 1.2TB so am I right to think its saying a max of 750GB HDD can be added?
What I really want to do is strip the HDD out and clone it onto like a 4TB drive then add another to the second slot and run raid1 at 4TB or more but I’m starting to question if I’m going to be able to do that at all.

Any help and or advice if you have or have had this unit to play around with would be great. Thanks

take a look at HPs site and pull the manual, should give you an idea if the box will support larger drives.