I was having the same problem of PM not bringing up the ingest dialog in El Capitan.
One can configure Image Capture to have PM open upon seeing a camera disk, but the Photos App would also always open it's import dialog box, and then I would need to uncheck a box to teach Photos to not open when "this camera, (or this disk)" is mounted.
I needed to do the same operation in Photos for disks from every camera, and also configure Image Capture for disks from every camera, and only then Photos would not launch and PM would launch.
However, if you format a disk in the camera, and I do that a lot, all is forgotten, and Photos opens again, Image Capture does not remember the settings and PM will not open the ingest dialog.
My unsophisticated solution to this problem was to delete the Photos App.
(El Capitan protects the Photos App and will not let it be deleted, so I booted into an external disk and in that way I was able to delete the App.)
And since then PM launches the ingest dialog upon insertion of a disk as expected, and my computer seems not to complain about the missing app.
I have no idea what happens under the hood, and what will happen down the road when El Capitan gets an update, and if the missing Photos App will cause problems, but for now I'm happy I don't have to fight El Capitan.