The only reason I'm posting this is because I was stuck for a few hours and the internet had tons of conflicting fixes for this that led me nowhere. Still stuck with GPT errors and a whole lot of general bollocks. For some reason this refused to work on the USB port on the left of my macbook and for reasons known only to Job's & Gates' lovechild, worked on the one located on the right. Sorcery.
To Dual Boot Mac OS X El Capitan with Windows 10 follow the below instructions. Choose the Language preferences with time, and Currency format then click Next to go forward. Choosing a Language. Now click on Install Now. Install Operating System. El Cap seems to have nuked my Bootcamp created Windows 10 partition and it won't let me install one outright. I get the 'This is GPT partition style' error, and I'm not sure how to fix it under El Capitan.
This is what worked for me, I can't guarantee that it will work for you and I'm by no means a Mac expert - so use at your own risk. Caveat emptor. No screenshots because of reasons.
What you need:
1. MacBook or other Mac Computer that is listed here: https://support.apple.com/en-us/HT204990
2. Windows 10 64-bit ISO.3. A fresh USB stick, 8G or more would be fine.
4. At least 100GB free on your Mac for a 64GB Windows install.
0. Update your mac if needed.
El Capitan Windows 10 Bootcamp Installer
1. Run bootcamp and select the two options- Create a Windows 7 or later setup disk
- Download the latest windows support software from Apple1.1. Click Continue, grab a beer. or three. This takes a while.
El Capitan Windows 10 Bootcamp Download
2. Exit bootcamp. Because of reasons.3. Fire up bootcamp and this time select the option to install (only).
4. Drag a partition size that suits you, I went with 64GB - then click Continue
5. Magic happens then the computer would probably reboot right into OSX. Manually reboot and this time hit the Option Key, Feel free to bang away at it. (Disclaimer: Do not bang away at it.)
6. When presented with boot options, please ignore the Windows option. Select UEFI boot.
7. Oh looky, run through the windows install until you get to the Disk screen. At this point choose your desired windows partition created in step 4 and then click format. The next button magically appears and you should now be able to proceed with installation.
8. Magic happens again, then a couple reboots later and Win 10 is done. Grab another beer. or three.
9. The next time you bring up the dual boot screen, Only OSX and Windows would be presented and the UEFI option is gone.
10. Happy dualbooting. Hell you'd be happy too after six beers.