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When you connect your device and Photos opens, uncheck the "Open Photos for this Device" button in the top left.

Screen Shot 2015-05-28 at 11.05.43 PM.png
 
I'm also having problems with this, despite checking the box as per the post above Photos still opens whenever I connect a card reader to my mac.
 
Then try doing the same thing in Image Capture. Start Image Capture, connect your device, select it and set the option in the lower left window. If you don't see it click the tiny icon down there to expand.
Screen Shot 2015-05-31 at 11.21.19 PM.png
 
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