iMac Mid-2011 21.5 (High Sierra) won't bootcamp Windows 8.1
I'm trying to install Windows 8.1 on my Mid-2011 21.5 iMac running OS 10.13.6. I gather that I need to use a DVD containing the Windows install disk. I don't have an original disk, so I downloaded the 64bit version of the iso file from the Microsoft website and burned it to a blank DVD using my internal DVD drive.
The problem is Bootcamp Assistant won't recognise the disc.
Inspecting the DVD using Disk Utility tells me that the DVD is not bootable. So I think that might be the problem, but I don't know why it wouldn't be bootable or how to change that. Any ideas?
iMac, macOS High Sierra (10.13.6)