I'm not familiar with the management interfaces of any of these products, but the "RAID stuff across multiple different size drives" phrase caught my eye. My understanding, from a friend who uses a Drobo NAS, is that most of them limit the RAID capacity to multiples of the smallest drive currently installed.

So when upgrading, by installing larger drives in place of smaller ones, the new larger capacities don't normally become available for use until all drives are upgraded. Then the management software simply does an on-the-fly "filesystem resize" (standard Linux feature) to add the extra capacity into the pool.

But no, I can't help with the original question, as I simply don't use "NAS" management stuff of any kind. Here, I just have a box running Ubuntu, and I add drives to it as needed, and it all just works without fuss. But I don't even use RAID myself -- too complex and the rebuild times are total insanity with modern TB+ drives. Instead, I just stick with periodic filesystem mirroring to a set of backup drives.

Edit: for a lot of people, I expect the "unRAID Plus" NAS software is a lot more appropriate than any kind of RAID when dealing with huge TB+ drives.

Edit2: Or even just their packaged hardware+software systems -- way more expandable than most others I see out there, and without the nasty RAID rebuild issues.

Cheers