Installing Windows 10 on iMac 27" (2017) Mojave, 3TB I'm trying to install Windows on my Mac. I had no problems before, or on my older iMac/Macbook, with installing Windows on a Mac. 

I decided to detach my Fusion Drive, so I ended up with a 3TB HDD and a 128gb SSD. That gave me more flexibility. The 3TB HDD is is formatted with APFS. In Recovery mode, I split the drive in 2 partitions. 750GB for Windows and 2250 for Mac Data.

Made a Windows Bootable USB with UNetbootin. With a downloaded Windows from the Microsoft.com website. 

Before I booted into the Windows-USB, I detached every single USB-port on my mac, even Ethernet. (I've been there)

In the Windows installation, when checking which partition you want to use it says; "We couldn't create a new partition.."

I tried this many times, everytime I changed something. Maybe it worked. But no. This is what I've tried so far.


Delete partition, Windows installer made a new partition of the unallocated space. Didn't work, same error.
Opening cmd, using diskpart. Tried to delete the partition, create new one an format in ntfs - and try again. Still no luck. Had also difficulties with formatting only the partition on the HDD. But when I (accidentally) formatted it, it formatted the complete drive. But still didn't work afterwards.


Use another USB-stick, 3.0/2.0, USB-port
Tried another Windows installation
Reset NVRAM and PRAM



I have no idea what to do anymore. I hope that here's a wizard who can tell me what to do!

Thanks for your time!
If the second partition on HDD is for Mac data, you could attempt to install Windows 10 by allocating the entire HDD to Windows. After you have finished installing Windows, you could then shrink the Windows volume and create a partition for the Mac data.