This is how I installed Windows 10 on iMac Late 2013 upgraded to 24GB of RAM. I used the Buffalo MiniStation Thunderbolt upgraded with Crucial MX 500 SSD external drive.
- Get Bootcamp Drivers using brigadier.
- Download Windows 10 ISO e.g. Win10_1607_English_x64.iso.
- Use ‘Quick Install Windows’ from Winclone 10.0.
- Select the Windows 10 ISO and the mounted WindowsSupport.dmg (Bootcamp Drivers).
- Select the destination thunderbolt drive.