How do I fix "Your Disk Could Not Be Partitioned" isssue with BootCamp Assistant?

I'm attempting to install Windows 10 on my iMac 27" (2012) with 3TB+128GB Fusion Drive, but during install procedure I get the same error "Your Disk Could Not Be Partitioned" without any further explanation. The first time I tried to install Win, I got stuck on Apple Support Software download, so I canceled and I downloaded the software on a pen drive myself. So I uncleared "Download Apple Support Software" and I only cleared the option about installing Windows. The first time the partitioning started, but it got stuck. So I closed BootCamp and I repaired the disk. Since that time, BootCamp never worked anymore. I tried any possible kind of repair and recovery, including:

• Internet recovery
• /sbin/fsck -fy from Single User Mode
• Repair using Disk Utility in Recovery Mode, which all said Your Disk appears to be OK!

I have also noticed a lag with Safari Smart Bar (I don't know if it has anything to do with this, but I'm totally sure that it shouldn't happen, because I have 16 GB of RAM and a 3.6 GhZ Quad Core Intel CPU, so it MUST have to do with the Fusion Drive.

Before closing this question I'd like to mention that my Fusion has been replaced with another for free due to extreme disk corruption https://www.apple.com/support/imac-harddrive-3tb/ Help will be much appreciated!

The following post (see hyperlink) was useful, especially the bit about how you have to create a New partition when you're in the Windows installer. My previous answer was downvoted and I deleted it, but this actually solved the issue for me. https://fgimian.github.io/blog/2016/03/12/installing-windows-10-on-a-mac-without-bootcamp/