Bseven,
Just a quick point here, I can't speak for the 747 (any type) but the 757/767 have separate smoke detector fans (2 per cargo bay/detector pair) to draw the cargo bay air through the detectors. Only one is running at any time. A plenum pressure switch senses failure of one of the fans and will automatically start the second fan, so a dual fan failure on this type will cause loss of smoke detection. This will of course produce the relevant EICAS status/advisory for QRH action. Bleed air is NOT the air source for the detectors as it may in itself be contaminated.