"You can select the APU GEN ON while still on external power, assuming the APU is running. It does however switch off the cargo system, which is bloody annoying since this involves resetting it."
Thanks... CR2.
So even though the Cargo was being powered by the default power source, selecting APU Gen 2 ON, still affected the cargo. Weird!
BTW, by "resetting", you mean simply pushing the APU Gen 2 button (back to AVAIL)... or is there something else which has to be reset outside the cockpit? (I may as well get the full story here whilst I'm getting all these excellent answers ;-)).
Cheers.
Q.