1.Create USB stick using Apple Bootcamp Assistant. This procedure only works with it.
2.When it Bootamp assitant reboots your machine you need to interrupt the installation.
3.When you hear the startup sound, hold option until it presents you with login options.
4.Select your Macintosh HD to boot back into OSX.
5.Run Disk Utility, select the Macintosh then choose the partitions tab.
6.The BOOTCAMP partition should be at the bottom. Select this partition by clicking on it. Ensure it is the highlighted partition then press minus to remove it.
7.Click apply on the bottom right to make the changes.
8.Now press plus to add a new partition which will likely be names Macintosh HD2
9.Select this new partition then you can define the settings on the right.
10.Rename this to BOOTCAMP.
11.Change the type to ExFAT.
12.Now click apply on the again to make the changes.
13.Reboot you Mac, press option again during the boot and this time select the Windows device with the USB logo on it. Its orange.
14.Now it will start the install process. When it gets to the list of partition you should see four of them. You want the last one. It will say it cannot install on this.
15.Make sure the fourth partition is selected then click format. When it is done the error message should be gone and you should be able to continue.
16.During install it will suggest turning on auto updates. I recommend you turn this off until the bootcamp drivers are installed. When it is finished prepping the install it will reboot.
17.Each time it reboots you will need to intervene by pressing option but from now on select the Windows device with the picture of the hard drive. It is a silver color
18.It will reboot two or three times during the install. Each time you will need press option during boot to reselect the Windows device on the hard drive.
19.At the end of the installation it will prompt you to install the Apple Bootcamp Drivers.
Article from: https://discussions.apple.com/message/23545484#23545484