Installation issue w/ Windows 10 on late 2015 27" retina iMac

I have the same model as you, and I've just spent the whole day trying to fix this, plus an hour on the phone with Apple support. Here is what I've learned.

Apple support agreed with me that the problem is most likely a bad display driver (AMD?) in the latest package of Boot Camp drivers. Unfortunately, Windows 10 compatibility for the late 2015 iMacs was only added last month, so there could easily still be bugs In the drivers. The result is that Windows can't turn on the display when Boot Camp drivers are running, but can when the default system drivers are running. This is why we could still install Windows and boot to the desktop at first (until Boot Camp installed drivers).

Support also suggested that the new El Capitan 10.11.2 update could have contributed to the bug, but wasn't sure about that.

In my experience, the following will not solve the problem:

• Resetting PRAM
• Reinstalling Windows (the bad drivers do not get removed)
• Using Windows System Restore
• Installing Parallels (you still can't touch the drivers)

The only remaining possible solutions thay we discussed on the phone are:

• Get into Windows 10 safe mode (very difficult, unless you hard reboot and reset PRAM repeatedly) and uninstall the graphics adapter drivers, or install AMD's latest drivers
• Delete your Windows partition, reinstall from scratch, and refuse Boot Camp driver installation when it comes up

If you try one of these solutions, please let us know if it works.

And if someone could please tell me how to guarantee being able to boot into Windows 10 safe mode, it would be greatly appreciated. For the record, F8 and shift-F8 are very unlikely to work.

This is not correct. What is the exact GPU on your iMac 5k? Are you using a USB to install W10, or the OSXRESERVED method (non-USB)?

Do you have the Radeon M380?

I looked further into this on my end. I have the Radeon M380, but through safe mode I discovered that Boot Camp installed the driver for the Radeon HD 8950. Clearly not the right driver.

Unfortunately, I couldn't install any alternate drivers from AMD. Uninstalling the driver made it impossible to boot into Windows even in safe mode.

1. If you already have W8.1 fully installed, and download the W10 ISO when running W8.1 and run the W10 setup.exe, you are running an upgrade. You already have W8.1 GPU drivers in place. These drivers are not compatible with W10 in general. W10 installer is supposed to switch to a generic video driver not dependent on the specific GPU. Most likely switching to the Intel GPU is where you are getting black screens from. Disconnect all external storage, hubs and run SMC and NVRAM reset. W8.1 must be fully updated before you run an in-place upgrade.

2. If running a W10 clean install (no previous Windows installation), the AMD driver does not come into the picture till BC drivers are installed, after you can log in and create an account. You should disconnect from the Internet after the Windows installation starts, otherwise you get updates from M$ which are WHQL drivers and these can cause problems. If the black screen comes up after the Account creation process is complete, it indicates a WHQL driver issue. BC drivers (from a USB or OSXRESERVED) must be in place prior to reboot otherwise GPU issues will occur.

3. You cannot put just the ISO on a USB and try to install W10, because a Windows PE (Preboot Environment) for a Mac does not exist. BC Assistant builds a USB which contains $WinPEDrivers$ which provides the proper GPU/BT environment to work. The same thing is done in OSXRESERVED.

4. Your 2014/2015 Macs are UEFI-compliant and can install W8.1/W10 directly from a W8.1/W10 installer (not an ISO) USB created on a Windows PC. If you unpack the ISO to a USB, you have problems as noted in #3.

You were exacty right on #2. I did a full reinstall without internet, and I saw the exact point when Windows tried to go online to search for display drivers - it was the exact same time as Boot Camp drivers were installing. There were no broken Boot Camp drivers.

Now everything works. Thank you.

Please backup OS X, Windows and create a Windows System restore point. I would also recommend disabling Automatic updates and manually checking Windows Update every week. Each driver offered by Windows Update must be carefully screened.

Windows Update should also get smarter vis-a-vis drivers and using Vendor ID and Product ID better. Apple's hardware variants need to be clearly documented to avoid such issue. This is a known issue since the time Apple officially started supporting Windows.

On the first Windows Installer screen, before you choose the destination partition, disconnect all network connections.

Download BC6 drivers using BC Assistant to a USB flash drive and switch to OSX and run setup.exe from the Bootcamp folder.

If you do not have USB reading capability, there is a workaround.

Thanks for the valuable input...you suggest not to automatic update the Windows update...

How to screen the Windows Updates that might impair the Boot Camp operation?

Please see http://windows.microsoft.com/en-us/windows-10/getstarted-choose-how-updates-are- installed and choose values as appropriate, other than Automatic.