Quick check:: Initially, you could check if the disc budges at all inside the slot when the drive is trying to eject it. Shine a light and take a look. This is to make sure the disc isn't bumping against the inside of the iMac's or the drive's casing. Thorough check:: Seeing as you already followed steps to remove a disc manually, you probably have taken the drive out of the iMac and opened the drive itself. I suggest running the drive while it is outside the Mac and with its (the drive's) covers off. I am not familiar enough with that model iMac to tell you how to do this. All you need is a way to power the drive outside the iMac and make it eject. Once you can see inside the drive, attempt to eject and observe what it is doing. Usually drives like these have a small motor that is tasked with lowering/moving the spindle the disc is sitting on and pushing the disc out. From your description, the motor is working, but some part it uses to push the disc with, might be stuck. Hope this helps.