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All screens go black and PC freezes when I turn off a monitor connected by HDMI. I can tell that it's frozen since NumLock does not toggle any more. Power-cycling the PC brings things back to normal.

Details: I have three Dell U2711 27" monitors connected. Two are connected via dual-link DVI, and one via 100 ft HDMI cable with a built-in signal booster by KanexPro. All three go through a Radeon HD 6xxx graphics card into a PC with redundant 750W power supplies.

All screens are running at 1650x1050, with one DVI-connected screen duplicated to HDMI-connected screen.

Latest BIOS and drivers throughout.

EDIT 100-ft cable has signal booster built-in. I connected input end to the graphics card and output end to the 27" LCD.

All works well with a short 3 ft HDMI cable.

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operating system? – MalwareManiac Dec 7 '12 at 21:42
Is it the monitor on the extender that causes the freeze? – David Schwartz Dec 7 '12 at 23:10
@MalwareManiac it's Server 2008 R2. – GregC Dec 8 '12 at 2:10
@DavidSchwartz please see my edits to the question. – GregC Dec 8 '12 at 2:11

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