If this was from a dedicated "NAS" box, then how many drives can the NAS hold? More than one, right?

In which case it probably always overlays a RAID of some kind onto the drive(s) before making filesystems. And doubly so if it used some kind of special hardware controller board for the drives.

What do these commands show you (run as "root" from within Ubuntu):

fdisk -l
cat /proc/partitions