OK, for those that come after, if there are any!

Yes the HDD size is limited to 2TB, however, if your drives are less than two TB they may still not work.

The limitation is that the drives need to be initiated with a MBR, not GPT. People who know more things than me explain it well here: https://www.howtogeek.com/193669/whats-the-difference-between-gpt-and-mbr-when-partitioning-a-drive/

Therefore I have a 2 x 4TB WD DUO connected to the router by USB, formatted to RAID 1 but only offereing 2TB of storage... but it works!

Cheers all, BarnabyS