Okay, a family member has a 2007 C2D 24" iMac that has a black screen, and since I supposedly know more about Macs it was given to me to fix. Problems here, as I am in over my head. The iMac is definitely starting fine. It shows nothing on the display, but I hear startup chime and can toggle the Voice Over on and off and determine it's running properly. I plugged in an external monitor with a known good adapter and there was nothing on the external display. This leads me to believe it's the GPU since the rest of the iMac seems to be fine. Can this GPU be replaced, and where do I even start? I read that using Target Disk Mode I could troubleshoot the iMac and determine the exact point of failure, but that's way over my head as I have only used it for file transfer. Please help. Are there any other tests I can do? How can I use TDM to my advantage? Thank you greatly in advance for any and all assistance. I am a Mac user, but the only iMac I have ever own is a current 27" that hasn't had a problem. I am very frustrated in my lack of education as to how TDM should help but Apple.com isn't helping much. Cheers.