16TB limit?

It creates a 6x4TB RAID-5 layer then tries to expand to add a 2x2TB RAID-1 layer using the remaining 2TB on each of the 6TB disks. That fails.

You can do a factory default (wipes all data, settings, everything), open RAIDar, click setup during the 10 minute countdown, choose Flex-RAID and confirm your choice. Delete the volume created by default and create the ones you want.

Another alternative is to put OS6 on your Pro units (also requires a factory default). However this is unsupported.

Now that's a bit of useful information!

So one with 6 x 6TB could be split up in 2 x 16TB (approx.) Flex-RAID volumes?

On 4.2.x you can create up to 4 volumes with any given disk included in up to 2 of these.

bartw wrote:
...I'm not expanding anything, these disks are all put in at once during a factory reset.

As I said, all disks need to be the same size.

Ixa wrote:
...As per StephenB's earlier response on this forum: StephenB wrote: You can create a larger volume size than 16 TiB if you use disks of the same size, and do a factory reset with them all in place. That volume can't be expanded though.

As mdgm pointed out, if the disks are not the same size, then creating the XRAID array requires an expansion step - even if you are doing a factory default.

Flexraid with two volumes (4x4TB RAID5 and 2x6TB RAID1) will give you 18 TB (12 TB in the first and 6 TB in the second). 6x4TB xraid would have given you 20 TB.

If you were to install OS6 on the pro, then you could get 22 TB, and future expansion would be possible. Given where you are, that might be the best option for you.