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.
El Capitan’s Boot Camp has USB drive-less Windows setup, but not all Macs are supported. Boot Camp, the Mac’s built-in software for dual-booting between OS X and Windows has been given some love in the new El Capitan software update and now features a slicker Windows setup that no longer requires a USB thumb drive, as discovered by Twocanoes. It once was a big selling feature for converting users who came from Windows. It lets you boot the Mac into Windows mode (from macOS), and go back to macOS at any time with a simple restart (from Windows). If you are running El Capitan (OS X 10.11), you can download the Boot Camp Support Software by following the steps given below.
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.
What's next?
Mac Os El Capitan Bootcamp Windows 10
0. Update your mac if needed.
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.
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.
El Capitan Boot Camp Windows 10
El Capitan Boot Camp Windows 10
9. The next time you bring up the dual boot screen, Only OSX and Windows would be presented and the UEFI option is gone.