H700 last disk Available RAID Disk Space = 0

Hi.
We have an R710 with an H700 controller with 5 physical disks in RAID-5 that we are swapping out to allow adding more space.  The disks have been swapped by using Offline/remove/replace with new unit/wait for Rebuilding to complete, then repeat steps for each disk.

The array currently has a single Virtual Disk in MBR mode - which we plan to leaving alone, then add a second Virtual Disk configrured with GPT to allow a greater-than 2TB volume with the remaining added space.

The old disks were 300GB each (all identical models either as DELL-branded or OEM Seagates).  The new ones are 2TB each - again all identical models bought in a batch: Seagate ST2000NM0045 Exos 7E8 Enterprise, 3.5", SAS.

The first 4 drives are all on Backplane Connector 0 (so ID as 0:0:0 through to 0:0:3) and all report space as;
Capacity = 1,862.50GB,
Used RAID Disk Space = 278.88GB and
Available RAID Disk Space = 1,583.62GB.

The last disk is on Connector 1 (so ID's as 1:0:4) reports the same Capacity and Used RAID space, but it reports the Available as 0.00GB.  So I'm being prevented from adding a new Virtual Disk, as the OMSA Wizard reports no free space.

Our H700 controller details:
Firmware Version 12.10.7-0001
Driver Version 6.801.05.00
Storport Driver Version 6.1.7601.18386

I have tried removing and reinserting that last disk, but after the rebuild it still reports the same zero available.
Could this relate to it being on the second Enclosure backplane (Connector 1)?

I'm planning a reboot of the system overnight just in case a hardware-level restart is required - but I'd welcome any other guidance!