Mark

Further explanation.....

The square opening at back of the operating blower is used for discharged air.

When a blower is running, its own flapper is being pushed back to let the air escape.

The non-running blower will have its flap pulled "in" because of the suction through the manifold.

We recently had a aircraft with a #1 blower failure (the #1 is the primary and is powered up first).
The blower failed which caused the system to use the #2.
The flapper on the back of the #1 was missing.
As the #2 was blowing, air entered the back of the #1 blower into the manifold.
No air was drawn through the detectors and the pressure switch was not being activated.
The system threw a EICAS message of "Cargo Det Air" because all smoke detection in that bay was inoperative.
  
Crappy image, but  it shows the location of missing flap for the #1 blower.
  
     
  
Replacement flapper.