I think I figured it out. Photo booth recognizes the existence of 6 capture devices but cannot bring one up.
Apple suggests it's USB Bus overload. They suggest adding a USB 3.0 port by using a Thunderbolt 1 to USB 3.0 hub.
Apple say Amazon and eBay has things called Mini Display Port hubs which could be interpreted as Thunderbolt 1. Apple says the only difference between thunderbolt 1 and 2 is capacity of data through ports. So anything that's a display port to USB 3.0 adapter should work.
I'll try that Sunday.
If the data ports have enough room, assuming I don't upgrade equipment until unworkability, and since currently I kinescope the screen and get my network from my cell, this will probably be the final upgrade until decent land internet comes in.
My main concern should be data bus traffic. What's a decent Mac Mini that can run 4 cameras at 640 x 480 and one video game at 1080p and enough web to run Speechchat.
