Creating a second volume on an EDA500?

OS 6.6.1

RN314 (two 4TB reds, two 2 TB Reds)

EDA500 expansion chasis (three 4 TB Reds, two 2TB Reds)

OK, I'm trying to create a third volume . . .  The 314 disks are one volume (RAID 5 - X-RAID), the three 4TB Reds in the expansion chassis are a second volume (RAID 5 - X-RAID).

I've inserted the two 2TB Reds in the EDA500 and they are recognized -- they show up as black on the System-Volumes page. I can select them and format them but cannot select them and create a new (third) volume.

Am I missing something or is this something not possible?

BTW: the EDA500 is awesome! Significantly faster for backups than standalone USB or eSATA drives.

Thanks!