in an attempt to move all my noisy drives to nearby closet and reduce noise at my desk, I just purchased a 15 feet FireWire 800 cable (http://www.amazon.com/gp/product/B0069VBAXU?psc=1&redirect=true&ref_=oh_aui_detailpage_o03_s00) I read online that FireWire was good until 4.5m (15 feet) but my drives do not mount any longer. Am I to assume the cable is broken/DOA or is this an issue of length? Needless to say my drives mount fine with my shorter 6 feet FireWire 800 cables
It could be either, but if you look at the customer reviews you will see a number of people have had problems with this cable due to the length.
At a guess, the cable is not a "top quality" cable and your system may even be very slightly out of spec (maybe voltage?) as well, so the cable is not working.
Its possible a higher quality cable will work.
If you want to check if the cable is broken/doa, you can always get a multimeter out and test each set of pins.