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Having tried to debug a really annoying 21.5" iMac for a day now, I'd appreciate any hints. The scenario goes like this:

We got an iMac 21.5"(Mid-2011) for an auditorium and plugged it in through an adapter contraption; (Thunderbolt)Mini-DVI to DVI, DVI to HDMI and HDMI cable to video projector. This didn't work with the iMac, but worked as expected on MacBooks, with and without Thunderbolt.

Port broken? Connect the iMac to a Apple Thunderbolt Display, works like a charm.

So the port works for Thunderbolt, but not DVI/HDMI? Testing different cables with an HDMI TV confirms it, HDMI doesn't work. The TV says a device is connected, but not transmitting. At no point has the iMac ever detected a secondary display, save for the Apple Thunderbolt Display.

Check firmware updates, PRAM, NVRAM, various hot plug and restart scenarios and eventually revert back to Snow Leopard. Nope.

What could contribute to this difference in behavior? Selectively broken port? Video card?

We'll likely need to test with another iMac and more adapters, but if anything comes to mind (the mysterious /System/Library/Displays/Overrides/ files?) I'd love to hear ideas to debug this sucker and just make it work ;)

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