ReadyNAS NV+ v2 Disk Expansion X-RAID2 Firmware: RAIDiator 5.3.11

With 4x2TB disks if any one disk fails data remains intact. Adding one 6TB disk if we allowed expansion the 4TB would only be stored on the one disk and would not be redundant. In such a scenario if the 6TB disk failed then data would be lost, but that goes against the principle of X-RAID withstanding a single disk failure so we don't allow that expansion. We would require two 6TB disks to be added to get vertical expansion, not just one. After the second 6TB disk is installed there could be 4TB of expansion in the form of a new 2 disk RAID-1 layer.

You can think of the capacity of an X-RAID2 volume as the sum of the capacity of the disks minus the capacity of the largest disk. So 4x2TB - 1x2TB = 3x2TB + 1x6TB - 1x6TB = 3x2TB, so there is no expansion.

4x6TB disks would lead to a volume capacity slightly over 16TB, so expansion adding a fourth 6TB disk would fail. I believe the NV+ v2 would not support volume capacities above 16TB even if you were to do a factory default.

With such high capacity disks as 6TB disks installed I think you would find things like an offline filesystem check would run out of memory and you may run into performance issues. Even using 4TB disks would be pushing the NV+ v2 pretty hard.

With 4x2TB disks your volume capacity would be about 5.5TB so you could expand up to about 13.5TB. So theoretically I would guess you should be able to have up to 3x6TB disks in the NAS. However I don't know whether the expansion would succeed or fail. 6TB disks in the NV+ v2 is an unsupported configuration.

If you use your NV+ v2 for primary storage of important data I would suggest you backup your data as the next step. RAID is great, but it's not a replacement for backing up your data (note if the primary copy of your data is on your PC, then the copy on your NAS is a backup). See Preventing Catastrophic Data Loss