How to make a Mac forget it ever saw an iOS device? I need this just to be able to test how the machine responds to the first time a device is plugged in - without needing an unlimited supply of 'new' devices.

I connected a 'foreign' iPad [my partner's, not synced to this machine but to her own] to test Photos app's response.  It auto-launched.  

Now I want to try again, let it 'see' the device for the first time - so I'm trying to discover what keeps the record of which devices were connected &amp; if I can clean it of just one device without damaging my regular devices' behaviour.

Closest I can find is How do I make iTunes 'forget' a device in Mavericks? which has some discussion regarding Sync Services - but I'm not sure if that's heading in the right direction.

Also relevant - How can I prevent iPhoto from opening everytime I plug my iPad/iPhone? - which is my eventual aim.
I'm not sure exactly, but I can help you figure out which files changed after you plugged in the device.  Hopefully you're comfortable with Terminal.

Start a timer.  Plug in your new device, treat it like you would any other new device - trust it, let it sync, whatever.

Once that's all done, stop your timer, see how long it took and round up in minutes.  Let's assume it took 5:18.  We're going to round up to 6 minutes.

$ find ~ -type f -cmin -6 &gt; ~/Desktop/files_modified.txt


Now you have a file showing all of the files that changed in the last 6 minutes.  Some of these files are going to be relevant and most aren't: when I ran this experiment I got lots of files changing related to Safari, for example.

Good luck, I hope that helps.