Brand new iMac, need formatting help

I Just recently purchased a brand new 27in iMac with a 1Tb Fusion drive. I used bootcamp to installed Windows 10 for some of my college classes and everything worked great! Well, after i ran Windows for a few days I decided that i needed just a little more space on my partition. I booted into OS X and using disk utility I gave Windows a little more storage in its partition. Long story short, Windows freaked out and ive erased the hard drive twice just trying to get Windows to install. Everytime I've tried to install Windows, it gives me the error "windows detected that the efi system partition was formatted as ntfs". Can somebody please give me some insight on what to do? -Ed