I'm setting up a brand new Late 2015 iMac 27 5K with a 512GB Flash Drive but I'm having issues installing Windows 10 to Bootcamp. It's a clean install of OS 10.11.1 and as such Bootcamp assistant doesn't require an external USB drive for the windows files. The assistant starts, allows me to create a 100GB partition and automatically finds my Win10 ISO but after downloading the Windows support files, rebooting and the Windows installer kicking in I reach the part where I choose the partition to install into. I can select the BOOTCAMP partition which is initially unable to install windows and Format it and then the BOOTCAMP label disappears. I then select the newly formatted partition and click next but then after a couple of seconds pause it reports: 'We couldn't create a new partition or locate an existing one' The only option I then have is to quit the installer which reboots me back into El Capitan. Running the Bootcamp installer only gives me the option to delete the Mac drive back into a single volume. I've been googling and others have had similar issues but most are referring to the old way of installing Bootcamp using an external USB drive. I don't have that option with El Capitan ! I've removed all USB devices aside from my mouse so it's not that. I know my ISO is fine as it worked with a Parallels install I had before I wiped the Flash drive to clean install El Capitan. Can't see any way past this ! Help !