If sdk mode is what I think it is, you need flasher. Ensure you are running flasher with privileges (administrator/sudo). Enter this code, connect phone, and it will set to r&d mode.
Disconnect, boot. You will see some extra info on boot screen.

To undo it, enter this in flasher, connect phone, then disconnect and reboot. It will all be back to normal.
You should attempt to run the camera in r&d mode and after you exit r&d mode just to make sure

I use this all the time when I break a css file and my homescreen stops working while I am developing themes :P