I had this but after upgrading from Celena to Daryna. Have you tried this?
Unplug the misbehaving USB key (might be an MP3 player too.)
Type gconf-editor; press enter.
In Gconf-editor, navigate to...
double-click the key listed in the right-hand pane and then remove the word "usefree" from the bottom of the list.
Plug back in your USB key and see if it now automounts.