Upgrade RAID5 disks from 2TB to 3TB doesn't resize

I have an EX4100 and started with two 2TB Greens and two 3TB Reds. It correctly built a RAID5 of 5.87TB. I have just replaced the two 2TB drives with 3TB Reds - one at a time - allowing the RAID to rebuild each time. While all is shown as OK - it still shows 5.87TB and has shown me no obvious way of upsizing to 9TB. It does offer me the option of making a drive from the extra space - but that is not what I want to do.
I am wondering whether I had created a drive from my “spare” 1TB per drive the first time around - but can see no sign of it to enable me to delete it.
So - how do I upsize the RAID to 9TB? Or do I just go for a 3TB RAID5 as a second volume?