I had a similar problem with my ATI card on the computer. I think that the HD621 doesn't give the computer an EDID signal that there is a monitor attached so the ATI card turns off the HDMI port (I am using Windows 7) I ended up purchasing a product from Gefen called and HDMI Detective Plus. It is basically a pass-through box with HDMI in and HDMI out that tricks the PC into thinking that there is a proper audio/video device attached at all times. That way you can power down you system without having the re-boot your PC. The device comes with some standard EDID codes on board that are selected by DIP switches or you can attach it to a monitor or AV device and it can record the EDID from that device (which is the only time it needs power, otherwise it operates without a separate power cable.) Don't mean to sound like a pitchman for Gefen but it did allow me to get my PC and HD621 to get along peacefully!

John

861v8, HD621, 7.1 analog speakers.