The NV+ v1 is limited to 2 TB drives, but you can install 4x2TB. That should give you 6 TB of storage (though since the NAS uses TiB units it will report something around 5555 GB). My NV+ has 4x2TB.
If the volume is very old, it might be best to make a backup of the data, update the firmware, and then do a fresh factory install with all disks in place. You'd need to reconfigure, and restore data from the backup. There is more info on why you might want to do that here: http://www.rnasguide.com/2011/06/22/why-you-might-want-to-factory-reset-a-sparc-readynas/
