The login screen to my iMac is now half black every time it starts as shown here. Once logged in, the display is fine and I can view the whole screen. :s
Any ideas why this might be and how to resolve this?
If it is like that under boot, login screen (as shown), and logged in then it is likely that there is a problem with the actual display or the cables that connect the display to the rest of the system.
It could be the video card as well. If you want to be really sure it is not software related try booting from an external drive, if you can.