Hi to all, and particular greetings to all those who have helped me in the past!
Since i got my Kobo Glo HD I have been having problems being recognized by my Mac on MAC OS 10 El Capitan (10.11.5 at the moment). This seems to be the same problem you are having, Tommass.
I have noticed that changing cables seems to help sometimes but this is not a reliable solution. Perhaps you might try it though.
I have noticed that if I have the patience to leave the Kobo Glo HD connected for quite a while (I mean for hours!!) it sometimes seems to be recognized and appears on the screen of the Mac and I can then sideload documents and books.
I should add that my KObo Aura HD is recognized without any problem, as is my Kobo Aura.
The flickering effect and the "incorrectly disconnected" messages which Tommass mentioned also appear irregularly when the Kobo Glo HD is connected but not when the other Kobos are connected.
The Kobo Glo HD charges (proving that it is indeed connected) even though it is not recognized.
I can't offer any other help, but I would like to know if other users have had this problem and found a solution.
PROBLEM SOLVED!
I opened disk utility, as indeed Josieb1 had suggested above (advice I had not taken into account — my bad!) and found that the Kobo was recognised in a faint grey mode. I ran SOS disk and the Kobo was recognized. I am not sure if this is permanent but it might be a solution for you Tommass or others: that is why I'm leaving this post, even though my problem is solved.
Best,
Rojofo.