You're reply triggered some thoughts and some exploration.
I had the USB Remote in OnePush mode. This was triggering a second shoot when I connected the power again.
So I changed the trigger to TwoPush mode. Now things work as expected.
That doesn't sound right.
With the patch I posted, CHDK thinks the USB 5V power is always applied. When you issue a shoot command over ptp, the shooting sequence starts and if USB sync is enabled, halts waiting for the USB power to go away. The code at that point polls the USB power directly - bypassing the hack I posted. When the power goes away, the shoot completes.
I think the problem you are seeing is with your setup. Set the USB Switch Type
and the USB Control Mode
. That should do what you want.