Basically, any type of NAS can work as an overflow if you wish, whether its Melco or Netgear or otherwise. They don't need to run UPnP, the Innuos will just monitor as a fixed indexed location.
Your unit can indeed be upgraded with an 8TB internal drive, but for cost etc you will need to contact a dealer local to your region.
Although a very different proposition in terms of cost etc, do note we have just launched a trade-in campaign where specific Innuos dealers will take back your existing ZENith Mk3 to get discount off a ZEN or ZENith Next-Gen, which can come with up to 16TB.
