Unable to install bootcamp as I cannot partition my HD
I am trying to install Bootcamp on my iMac that has a 3TB fusion drive. When installing Bootcamp it fails as it says it is unable to partition my HD. I have followed the instructions on Apple's help pages. I am trying to sort the partitioning on my HD in Disk Utility. It is listed as Fusion Drive>Container Disk2>and under that I have Macintosh HD (11.16GB used) and Macintosh HD Data(1.16TB used) In the top right of both of these it says 3.12TB, shared by 5 volumes.
These are both clearly the same physical drive. When I try to remove there space from Macintosh HD - Data it seems to think the HD is 6tb, which it isn't.
When I click on Free Space and then the "-" button, this is what shows up:
What do I don now, as if I press Apply, it then asks me to partition and then fails. Any help would be gratefully received.
Thanks
iMac Line (2012 and Later)