Hi Brenden,
Many thanks for replaying my request. As I see on the links You sent to me, my iMac doesn't have a integrated and a discrete graphic card. Only a discrete graphic card is present, if I understand right. But, I think, I have to tell You the whole "story" about my difficulties.
One day, starting my iMac, it hangs after doing the start sound and showing the Apple logo and 1/3 of the progress bar. After about 3 minutes, the mouse pointer appears (movable) at the left upper edge in the gray screen. --- And thats it. No further reaction for hours.
I found, if I start in safe boot mode, I get the login window as before. Just the screen flickers a bit and scrolls 3 times itself. After this, I can log in and work nearly as before. Some restrictions I have now in this mode. The speakers, the micro and the camera (in Skype) don't work. Facetime and TeamViewer brings up the camera as before (strange, isn't?)
Prior to this issue, I had a bootcamp partition with Win 8.1 on it. Since the system was hanging, the win partition was grayed out. So I couldn't choose it to start with. Then I decided to erase the whole HDD and did a clean install with Yosemite.
Thereafter, an expert from the support community said, the graphic processor might be defective. I had to send him a system check dump, generated by a UNIX script, he gave me. (see my former post : Yosemite 10.10.1 will not boot regularly)
Now, my idea is, to reactivate the units which disabled the safe boot mode, I suppose. I don't know, if I'm right with my idea. But, this way I could work with my iMac nearly normal. Maybe, there are other reasons for the faulty functionality on my iMac, who knows?
Many thanks for Your help in advance!
Regards,
Supportix