You almost certainly have the v1 (silver chassis) which as mentioned does not support 3TB disks (see Limitation #2 on http://www.readynas.com/RAIDiator_4_1_8_Notes
). The v2 (charcoal chassis) would not run into this problem.
I would suggest if possible you return the NV+ (v1) and either get the NV+ v2 or get a ReadyNAS Ultra 4. The v2 and the Ultra 4 both support 3TB drives.
The NV+ (v1) is no longer in production and is disappearing off the shelves.