An incomplete unit is a unit in which the 3ware RAID controller is unable to detect one or more drives. The drives may be missing, dead, or improperly connected. A unit that is incomplete is also degraded (although a degraded unit can be complete if all drives are still detected, including the failed drive).
Check hardware connections and reseat the drives. Rescan the controller from 3DM 2 to see if the unit has been restored. If you are able to restore the unit before any data has been written to the unit, a rebuild will not be necessary. If the unit remains incomplete, replace the missing or dead drives and initiate a rebuild.