Boot Camp Assistant does not let me change my destination drive I received my new iMac today and want to install a Windows 8 partition to allow me to test software I'm writing with Qt on both OS X and Mac (Requirement for uni module).

I have a Windows 8 ISO and opened Disk Utility then created a 250 GB FAT partition to hold the Windows file system.  Upon launching Boot Camp Assistant I encountered a strange problem, the only destination disk I could select was my 32 GB USB stick which I have plugged in.

Why is my new partition not visible to the Boot Camp Assistant software?
Best way to do this, delete the FAT partition and re-expand your HFS+ partition to fill up the space.

Use bootcamp assistant to create a bootable USB key (you'll need an 8GB one). It will ask for the ISO to make the installer.

Once it is done boot from the USB key and follow the prompts. Bootcamp will partition your drive for you at the beginning of the setup process and set it up properly to dual boot.

My guess is that something in the way you made the partition (location, type, etc.) was incorrect. If you let BootCamp do it, it works great.