Here's a tough one for you. I installed an SSD in the optical drive slot of my 2009 iMac a few months ago. It's the system drive, working in conjunction with the old internal hard drive. On occasion, with no clear pattern that I can detect, a reboot will present me with a blank gray screen (the linen background), with no icons to choose a User login (which is my setting — it's not defaulting to the two boxes to type in your name/password, either).
On this blank screen, if I hit the letter "D" for my username and then type in my password, it will log me in as usual. However, almost always after such a login, there will be oddities in the visual display. Sometimes, rarely, half of the screen won't show anything at all. At other times, more commonly, things will look normal, except that when I open Messages app, the buddy icons for all of my chat buddies will be missing.
So the issue seems to be related to screen drawing, but in a very haphazard way. Missing user icons are always part of the issue, but sometimes it goes beyond that. Has anybody else encountered this strange phenomenon?