A multi-unit blood processor uses a hydraulic bladder system to express blood components out of whole blood bags and into collection bags. A dummy whole blood bag, containing an inert fluid, may be placed in a processing location. The dummy bag has characteristics that allow the blood processor to recognize the presence of the dummy bag. An opaque feature may be detected by a photo sensor. A radio frequency sensor may be used to detect a passive radio frequency emitter on the dummy bag. The dummy bag may have a bar code label identifying it as a dummy bag. The blood processor will not attempt to process the dummy bag.