Windows 10 unable to install on partition formatted by Boot Camp Assistant
Your best course of action would be to install Windows 10 the same way Apple instructs you to install Windows 8.1. For these instructions, see the Apple website: Use Windows 8.1 on your Mac with Boot Camp. When using these instructions to install Windows 10, you might want to consider the following.


You need to burn the Windows ISO file to a DVD. Some Windows 10 ISO files are large enough that you will need a double layer (DL) DVD. Older versions of OS X used the Disk Utility application to burn the contents of an ISO file to a DVD. With the newer versions of macOS (OS X), you can burn the contents of an ISO file to a DVD directly from a Finder application window. You use the Boot Camp Assistant to download the Boot Camp Support Software to an external drive.
You already have a BOOTCAMP partition. Assuming your external drive installer has the correct Boot Camp Support Software, you should be able to install Windows 10 with using the Boot Camp Assistant. Insert the DVD and connect the external drive, restart the iMac and then immediately hold down the option key. When the DVD icon labeled "Windows" appears, select the icon then the arrow below the icon. If during installation, you boot back to macOS (OS X), goto Startup Disk under System Preferences. Select the Windows partition, then restart the iMac to continue installation.


If you do not want to use a DVD, then you can use the accepted answer to the question: How to install Windows 10 into a 2011 iMac without using the Boot Camp Assistant, an optical (DVD) drive or third party tools?. If you are using OSX 10.11.6, then you can skip step 3.



To be clear, Windows installations on this model Mac must use the legacy BIOS boot method. This requires the installation drive to contain partition entries in both the Master Boot Record (MBR) table and the GUID Partition Table (GPT). Windows will only see the MBR table entries and macOS (OS X) will only see the GPT entries. Therefore, Windows with think the drive is MBR partitioned, while macOS (OS X) will think the drive is GPT partitioned.