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I have a work-issued Dell Precision 7720 with a Nvidia Quadra P3000. I have tested that all of the monitors work.

The laptop remains closed as I only use external monitors. I have a 4K60 LG monitor connected to the laptop using a HDMI port (v1.4 based off the manual) and an HDMI cable. I have a second 1440p60 monitor connected to the laptop’s mini-DP port using a mini-DP to HDMI converter and then using a HDMI to HDMI connection to the monitor.

I recently bought a third portable monitor that is 1080p60 that can be powered and displayed by connecting to the USB-C port on the laptop using a USB-C to USB-C cable.

The issue is that when I plug the portable monitor to the laptop, the second 1440p60 monitor disconnects. It doesn’t have any signal and I cannot identify or find the monitor in the Windows 10 settings. When I unplug the portable monitor, the display comes back.

What is the issue I am facing and how do I resolve it?

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    Your video card has a maximum number of pixels it can present at one time. You've gone over that. However, as this is a work-issued computer, you need to be asking your work IT staff this question. Commented Feb 27, 2021 at 2:29

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What is the issue I am facing and how do I resolve it?

My guess is that you are running into the limits of the GPU. A laptop that's able to drive three external independent displays is rather rare in my experience. Having three video outputs does not necessarily mean it can drive three displays at the same time. I believe I have a good sized stack of video cards and laptops with three video outputs but can only drive two of them at a time. Why have three outputs if only two will work at a time? It's likely to save the users on needing adapters.

For a laptop owner this can be handy as it allows plugging in a keyboard, mouse, power supply, and two displays (one HDMI and the other mini-DP) without needing a dock, and when at one's desk the laptop can use a single cable to a Thunderbolt dock to get an external GPU, USB, Ethernet, power, and audio.

I did a quick look on the internet on just how many displays the laptop will support on its own (as I suspect you did as well) and found no mention of supporting three external displays. Again, in my experience support for three external displays is rare on laptops. There's often support for three displays on the GPU in these laptops but one of the video outputs will be hardwired to the internal display. The other two will be wired to the video ports on the side of the laptop. This often comes with some limitation like you've seen where one DisplayPort output on the GPU can switch between the USB-C port and the mini-DP port.

What makes things more confusing is that some laptops will support only two video outputs being live at a time but still support more than two displays. This is done with MST, multi-stream transport. A single DisplayPort output could be capable of driving two displays with a MST splitter or a MST capable display daisy-chained to another DisplayPort display.

To resolve this I'd recommend contacting your company help desk and/or Dell support to verify my theory that this laptop supports only two external displays. If they tell you otherwise then listen to them and not some random dude sitting in his mother's basement wearing his pajamas in the middle of the day. If the professionals can't resolve this then this pajama wearing rando thinks a Thunderbolt external GPU will resolve your issue. External GPUs are not cheap so rule out the other options and think about how badly you need that third display before spending what is likely to be a good sized chunk of a kilobuck to buy one.

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The number of displays supported may be limited by the GPU or integrated graphics. You may be able to work around the limit by adjusting how the the graphics are routed or the GPU/IGP is allocated.

  1. Reboot into BIOS (Win+R shutdown /fw /r or Press F12 while rebooting)
  2. Navigate to Video screen options
  3. Adjust the Switchable Graphics setting

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