Fixed my error!
VM -> Edit Settings -> Options -> Guest Operating System
I had this originally set to "CentOS 4/5/6/7 (64 bit)" [force of habit - we have many vms of this type at work]
Shutdown vm, modified to "Ubuntu Linux (64 bit)", booted and the errors disappeared.
This is booting via plop, first USB device attached is the unraid USB flash drive, second USB device is blank (& unmounted).