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 3DM2 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.