I think I've trawled the Internet for the last few days and haven't gotten any closer to my goal, to install Windows 10 on the 3rd partition of an old MacBook Pro (15-inch, Mid 2010) that I use for testing.
Problem 1: Windows ISO too big for a DVD So I first downloaded the Windows 10 from MSDN and proceeded to make an install DVD. Once downloaded, while MS says it's 4.6GB - it's clearly 4.8GB so I can't make a DVD (I don't have any DVD+R DL media). So I try the ISO to USB. Old Bootcamp Assistant (like from Yosemite) can make a bootable USB from the ISO, but it doesn't work right as BCA is running on different hardware. New Bootcamp on El Cap or Sierra won't make a bootable USB from an image period. Tried Disk Utility, hdutil and dd of which it just immediately fails when trying to write the USB. I then tried the Windows 7 ISO to USB utility from Microsoft (running inside VMWare) - and it at least got the ISO restored to the USB, however it's not bootable. The Windows Install USB drive I did make is mountable under Windows in VMWare and macOS 10.9 - 10.12. It's just not showing up as an option to boot to when holding the option key at boot.
Problem 2: Bootcamp can't seem to create a 3rd partition on my drive from the free space to install Windows Launch BCA, it does its thing by building a driver disk for 20 minutes and then goes to prep a partition. It sees 2 partitions and no free space. Even if I go back and create the partition BCA doesn't see the partition and no other partition is marked usable.
So I'm at the nexus of not quite sure how to proceed.
Has anyone added Windows as a 3rd partition on a Mac? How did you do that?
How are folks creating Mac bootable Windows install media?
Any advice much appreciated.