The NitroAV boxes I referred to are made by Stardom.
This is what I got from NitroAV tech support:
SATA only supports one volume at about 2.2TB maximum.
USB only supports one volume at about 2.2TB maximum.
FireWire800/1394b allows two volumes each about 2.2TB maximum.
SCSI is essentially unlimited in volume size and can support 15 or 30 volumes depending upon the Host Bus Adapter ~$100.
SATA and FireWire can be accessed concurrently which allows up to three volumes (one SATA, two FireWire).
In a nutshell with 5 drives you're limited to 1.1TB physical drives unless you use the SCSI interface. The boxes come with SCSI or SATA/USB/FW so there isn't a migration path if you start with SATA...although you can remove drives from a SATA box and install them into a SCSI box w/o data loss.
It might be possible to have multiple volumes, say 5 for example, but you'd only have access to two at any one time via FW. I haven't explored this, however.