Just built a PC with a Sapphire Radeon HD 5770 card. Using the Displayport cable to connect to a 2009 model 27" iMac. As soon as I switch to Target Display Mode on the iMac the PC screen starts flashing. The Mac's screen goes black indicating that it's "switched". What I've tried so far: - originally had Windows automatically download the driver for the board - installed the driver from the CD that came with the card. - start the machine with just the iMac hooked up - use most possible combinations of the cards two DVI ports for the 1st display and the HDMI port (card has 2 DVI, one HDMI and one Displayport). - did a a totally clean install of the Windows 7 64-bit, wiped the hard drive completely. Did not install any additional drivers to the machine other than the ones that Windows 7 itself installed - so the machine is completely "stock", then downloaded the latest driver from Sapphire's website. I put that driver on a flash drive and installed it and rebooted. A few times the Mac's screen has turned blue momentarily. Sometimes the display properties on Windows will show the second display as being detected. I called the company that sold me the cable. The rep immediately diagnosed it as a bad cable and sent me a new one. The new one behaved exactly the same. I have a ton of computer equipment and accessories lying around. Unfortunately very few Displayport devices. I do have a MacBook Air (fall 2010 version) and a MacBook Pro (fall 2008 model). I don't have a mini Displayport to mini Displayport cable to test with, but I may go buy one at the Apple store. I have a regular Displayport to Displayport cable that came with a client's Dell monitor. I may ask him to borrow the monitor. At this point I don't know if the problem is with the iMac, the cable, the PC graphics card or the PC I bought the cable from Circuit Assembly based upon a comment in a review of another cable on Amazon that a lot of reviewers were saying didn't work right at all. CA seems to have a pretty good grasp of this cables usage. I bought the 5 meter version, which I suspect may be the culprit (but the guy at CA dismissed that as a possibility). Here's the cable I bought: http://estore.circuitassembly.com/products/Mini-Displayport-to-Displayport-Adapter-Cable.html Anyone have any thoughts?