Possibly, intel NUC DN2820FYKH on Windows7 or 8, When not connecting display,Graphic Driver can't work.
Similar,on Ubuntu12.04 Server and Desktop, When not connecting display,Graphic Driver can't work.
Framebuffer device seems to need working on Graphic Driver.
How about plugging the "HDMI Dummy plug" on HDMI connector?
I would like to inform you that I just tested a DN2820FYKH NUC running Windows 7 and the latest BIOS 0047. I turned off the system which was connected to a monitor using the HDMI port.
Then, I disconnected the HDMI cable and restarted the NUC. I waited for a few minutes in order to plug the monitor again. It worked without problems.
Please remember that Intel does not test and validate our products on Linux, however we know that a lot of customer are using this OS successfully on NUCs and Intel Motherboards.
I understand that many people successfully run Linux on Intel Hardware - I've generally had very good luck with Intel motherboards as well.
However, it's clear from the traffic on this forum that there have also been many problems getting the the video to work correctly on the NUC family, including headless booting with the DN2820FYKH.
Did you test Win 7 in UEFI or Legacy boot mode? Did it come up with the correct video mode, or did it have reduced resolution?
I did some more testing and found that the NUC I was testing with would no longer boot with no monitor plugged in. I re-flashed BIOS 0047 (the same version I had been running before) from the BIOS-based BIOS updater, at which point it would boot with no monitor. I made no other changes, and the BIOS settings had not been reset by the BIOS update. I unplug the monitor, turn power on, wait about 15 seconds, plug a monitor in, and I see it sitting at the GRUB boot prompt. I can boot all the way to the Linux command line with no monitor plugged in, although the video mode at that point does not match the monitor. It looks like the Linux i915 driver has no ability to rescan the monitor after the driver is loaded, so changing the monitor after boot will give the wrong video mode.
I did a test with Windows 7 and found that it did boot, although not well. I had to enable legacy boot to boot the only Windows disk that I have, and the video was scrambled when I plugged the monitor in at the "Windows Error Recovery" menu - it was slanted sideways and only part of the video was showing on the monitor. If I waited until Windows came all the way up before plugging the monitor in then I found it was at lower resolution than if I booted with the monitor plugged in.
ThuJones, I have both UEFI and Legacy enabled and I did not have problems with the resolution on my monitor. Everything came up as it supposed to.
You can try testing the NUC on another monitor or while you are Windows try installing the latest graphics drivers available. Restart the system and try again to see if the behavior persists.