External monitor won't allow windows to boot on 2017 iMac Radeon 560

Boot into macOS, connect the external monitor using the cable, switch to Windows. What errors do you see/get? What is the brand/model of the external Monitor?

From iMac (21.5-inch, 2017) - Technical Specifications,

Video Support and Camera

• FaceTime HD camera
• Simultaneously supports full native resolution on the built-in display at millions of colors (21.5-inch) or 1 billion colors (21.5-inch 4K) and:
  • One 5120‑by‑2880 (5K) external display at 60Hz with support for 1 billion colors, or
  • Two 3840-by-2160 (4K UHD) external displays at 60Hz with support for 1 billion colors, or
  • Two 4096‑by‑2304 (4K) external displays at 60Hz with support for millions of colors
• Thunderbolt 3 digital video output
• Native DisplayPort output over USB‑C
• Thunderbolt 2, HDMI, DVI, and VGA output supported using adapters (sold separately)

There should be no other adapters and/or cables between the monitor and the iMac that carry any AV signals. If the monitor has a built-in USB hub, it should be separately connected to the iMac via a USBc cable.

TrabajadorSocial wrote:

Here is the monitor connection:

iMac -> Thunderbolt 3 to miniDisplayport adaptor -> miniDisplayport to Displayport cable -> Monitor's Displayport port.

Can you a link to the exact adapter you are using? The daisy-chain method is very unstable and does not work properly most of the time.

TrabajadorSocial wrote:

Problem 1: Windows won't fully boot up if the adapter is plugged into the Thunderbolt 3 port.

Problem 2: If I hot connect the adapter after boot-up, often the monitor displays for a few seconds and then disappears.

I use a USBc-to-DP cable - Amazon.com: Cable Matters USB C to DisplayPort Cable (USB-C to DisplayPort Cable / USB C to DP Cable) Supporting 4K 60Hz… . The USBc-to-TB is not the same as USBc-to-miniDP.

I'm seeing the same problem, but in my case a 27" 5k iMac and Radeon 570, and I'm using an old Epson projector. i've tried three adaptors, USB-C to VGA, and two different brands of USB-C to HDMI.

As far as I can tell, the AMD graphics driver crashes when the external monitor is plugged in. If that happens while booting, I end up at the Windows 10 troubleshooting screen. Unplugging the external monitor allows it to boot, but then plugging it back in results in everything working for about 10 seconds, followed by the external display going dark. There's nothing of note in the windows log files, although I might be missing something.

I can get it working by right-clicking the desktop (after the display goes through it's initial 10-seconds on, then off cycle), opening the AMD settings app, and waiting about 15 seconds. The external display usually comes back on and stays for good. Sometimes it takes a second try.

I've tried using the modified AMD drivers from bootcampdrivers.com, but those do not help. Removing the AMD drivers completely (which I did by re-installing windows before I realized what the issue was) does help - everything worked perfectly until I reinstalled the boot camp "windows support software" (which includes the AMD drivers).

TrabajadorSocial wrote:

This is probably a stupid question, but do we need to re-install the windows support software from bootcamp if we reinstall windows?

It depends on how you reinstall Windows. I did a full reinstall, so wiped out my applications including the original "windows support software" - so in my case I did need to reinstall that for full boot camp functionality, Apple Software Update, Boot Camp Utility, etc.

Also, when you were running without the bootcamp-sourced AMD drivers, what driver was running to power your external monitor--was it the generic windows video driver?

Yes it used the default generic windows video driver. I didn't really play around with it long enough to see if that had any drawbacks, but i'm also using an old epson projector that's low resolution by today's standards (it has an HDMI port, but it's not "HD").

Do you have Screen-Split enabled on the monitor?

Run a SMC Reset and test. Also, check the Windows Events for any events related to GPU or Display.

Does the external display work properly in macOS? Look for any GPU-related events.

Can you check if Windows Fast Start is enabled?