Cant un-mount disk during Windows install

I am trying to install Windows 8.1 on my iMac, currently running Yosemite.

I have the installation media for Windows on USB (this is because I downloaded the Student Edition, which is not supplied on a CD), so I cannot use Boot Camp as it does not accept USBs for installation, only CDs.

I plan to boot into the USB installation media and run the installation of Windows from there.

In trying to partition my drive (to form 1 half for IOS, 1 half for Windows), I boot into recovery mode and enter Disk Utility and arrange my partition. Unfortunately it comes up with the message Partition failed with the error: Couldn't unmount disk. I choose the GUID arrangement and 50gb for Mac Journaled and the remainder for MS DOS FAT with the intention of installing Windows there.

I have tried to force un-mount the drive using Terminal, and it tells me the drive is already unmounted or to use another command.

I have tried to reinstall Yosemite on the existing single drive to see if this helps, but this installation failed with 1.5 hours to go. My iMac will now no longer boot into Yosemite, only onto the Recovery Mode. I cannot boot into Yosemite at all and in trying to reinstall it from Recovery mode just comes up with a failure.

Therefore I need to wipe my disk, correct? But when I try this it wont let me un-mount the disk and I do not know which step to take next.

Please offer any suggestions, as in trying to install Windows I have now lost both Windows and IOS! I can only access recovery mode.