I think silverfox will have nailed it with his reply and I only had 4TB HDD's to test with. Let me check with ASUS."supports a maximum of 16TB (terabytes) of storage."
Doesn't it recognise HDDs over 4TB?
At the time of review, which was quite a bit ahead of the UK release, there was no PLEX server app to test with but there is now. I can't therefore comment directly but given the performance in the AV tests, I would expect it would make a very good PLEX server as it's certainly got the chops for almost all the transcoding you'd need.