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I have three screens connected to a Radeon 6850 card. Two are connected via DVI and the third is plugged into the DisplayPort using an active DisplayPort to DVI adapter. The adapter has a USB cable that plugs into the motherboard to give it power.

Strangely, all three displays work perfectly fine if I don't plug the USB power cable into anything. Why is this, and will it stop working if I stress the graphics?

Also, for some reason if I do plug the USB power cable in, the DisplayPort screen dies whenever I plug my external HDD in. I thought this was power issue - but without the DisplayPort power plugged in everything (including the HDD) works fine.

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I'm guessing this is a dual link DVI adapter, but you're not running at a dual link resolution. There are lots of single link adapters that don't need extra power - these are able to draw enough power from the DisplayPort connection itself on the GPU. So, I'd guess it's able to operate alright without the USB connection until you try to hit really high resolutions. I can't imagine it will cause a problem but there's no way to guarantee that without checking with the manufacturer of the adapter.

Regarding the hard drive issue, I'm guessing this is a grounding problem with the adapter (I've seen similar behavior from poorly made DP to miniDP adapters and traced the problem to a pin not being properly grounded out, this was endemic to most manufacturers for a while.) It could be something weird with the USB controller too, but that seems less likely.

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Thanks. I'm only running 1280*1024 on each monitor so that is probably why. It may be a grounding issue (it works fine if I put a thumb drive in, but dies with the external HDD). If I plug the adapter's USB power into my mobile phone's wall charger (5V 1A USB wall charger) instead of the computer is this safe, and might it solve the grounding problem? –  Andrew Nov 8 '12 at 22:57
    
I'd guess it's a problem on the DisplayPort pinouts, not the USB pinouts, so probably that wouldn't help. I'm just conjecturing, though, and it certainly couldn't hurt to try. –  Shinrai Nov 8 '12 at 23:24
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