Adding a disk to ReadyNAS Pro for RAID5/6 volume expansion?

Ok, I went ahead and, one by one, removed and reinserted the four original disks. Each time the two volumes present were properly rebuilt, and all partition tables now start at sector 64.
Nevertheless, I still cannot add the free space on the new disk 5 to the second volume. The partition tables of the old and new disks are identical,
Disks 1-4:

First usable sector is 34, last usable sector is 3907029134
Partitions will be aligned on 64-sector boundaries
Total free space is 2157 sectors (1.1 MiB)

Number  Start (sector)    End (sector)  Size       Code  Name
   1              64         8388671   4.0 GiB     FD00  
   2         8388672         9437247   512.0 MiB   FD00  
   3         9437248       442189887   206.4 GiB   FD00  
   4       442189888      3907027007   1.6 TiB     FD00  

Disk 5:

First usable sector is 34, last usable sector is 3907029134
Partitions will be aligned on 64-sector boundaries
Total free space is 3464839277 sectors (1.6 TiB)

Number  Start (sector)    End (sector)  Size       Code  Name
   1              64         8388671   4.0 GiB     FD00  
   2         8388672         9437247   512.0 MiB   FD00  
   3         9437248       442189887   206.4 GiB   FD00  

When I remove and reinsert the new disk 5, which is part of volume C, but not D, volume C is correctly rebuilt, and during that time, there is a grayed-out option in the tab for volume D to add this disk.
But: after rebuilding C finishes, this option is no longer there :(
The tab to create a new volume also shows available space on disk 5, but I can't figure out how to add it to an existing volume.

So, how can I convince Raidiator to reconsider this disk as 'new', and use its free space?

Has anyone else tried to add a new disk to a system with two RAID5/6 volumes, and it worked?

As a second-to-last resort, I might delete the second volume, then hope that I can recreate it using all five disks.

Roman