I for one am a little bit peeved at the fact that with the new release of OSX, Apple decided to automatically open Photos anytime you attach a device with some sort of inbuilt storage. Case and point, lately a lot of my day to day tasks involve updating iOS & Apps on a large number of iPads via Apple configurator which connect to a mac via USB. There can be upwards of 30/40 devices connected at one time. Initially i was going through the process of unchecking each tick box next to each iPad in the Photos.app which is not only mind numbing, its quite time consuming.
I went on a hunt to find a way of disabling this, initially I didn't find much however i managed to come across this fix. Which requires using terminal to execute a command to disable the hotplug setting when attaching external memory cards/storage.
To do this Finder > Applications > Utilities > Terminal.app
Once in terminal, execute the following command, and hit Enter
defaults -currentHost write com.apple.ImageCapture disableHotPlug -bool YES
Once complete, it should take effect immediately without OSX requiring to be shutdown/rebooted.
Hope this helps!