Making macOS High Sierra 10.13.1 work with 2 displays again

I have a Mac Mini (“Late 2014″) hooked up to an Asus PB278Q 27” display and a Dell 2405FPW display. This setup has served me well for two years, but today the Mac stopped talking to the Asus monitor for some reason. After some fiddling, I was able to fix it.

The usual process-of-elimination didn’t leave a lot of options. This hardware setup has not changed for 2 years, and always worked. So the notion of a bad cable or hardware failure seemed unlikely. Still, I tried changing the connections around a bit: instead of using a Mini DisplayPort to DisplayPort cable, I tried a Thunderbolt-to-HDMI adapter and HDMI, and that didn’t work either. The older Dell monitor supports DVI-D as its best connectivity option, so I borrowed that cable and tried it out on the Asus monitor. That actually worked, but only at 1920×1200 (I think; it might have been 1920×1024) rather than the full 2560×1440 resolution.

I fortunately also have an older Macbook Air sitting around, which also has a Mini DisplayPort port, and is running macOS Sierra (one version older than the recent High Sierra update that I installed on the Mac Mini). I hooked that up to the Asus monitor using my DisplayPort cable and it worked immediately.

So at this point, I knew that the Mac could display video to the Dell monitor and to the Asus monitor via the Mini-DisplayPort-to-DVI adapter and DVI cable. I knew that the Macbook Air could talk to the Asus monitor at full resolution using the DisplayPort cable. So it’s not the cables, and it’s not the display. That leaves the Mac Mini as the likely culprit.

Now, I haven’t changed much about the Mac Mini in the last year+, except for adding a USB KM switch since I now have a gaming PC also connected to the Asus display via HDMI. So my suspicion was that the High Sierra update caused the problem.

Some googling of queries like “high sierra display doesn’t work” found people griping about losing their 2nd monitor support after the High Sierra update.

I decided that it was probably software, maybe hardware, and that I should reinstall the OS and possibly wipe the disk and do a clean reinstall before bringing the Mac Mini to Apple for service. Of course I have a daily full disk backup thanks to SuperDuper and a second full disk backup to a local file server that gets copied to via borgbackup/borgmatic, so potentially wiping the Mac Mini’s internal drive is only a matter of a temporary inconvenience.

Also, of course I always buy the AppleCare 3 year extended warranty, because fixing Macs yourself is not really a thing (especially for laptops!). So if this ended up being a hardware issue, that’d be another matter of temporary inconvenience while I waited for a repair or replacement.

Fortunately, just rebooting with the macOS recovery key combo to reinstall the latest version from the network, which is Option-Command-R, got it reinstalling, and after it rebooted, everything works. Woo hoo! My hard disk was not erased in the process, so there’s no need for the additional time required to restore a backup.

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