apologies if this is inappropriate. I've posted this twice on r/applehelp on reddit and got no help, so I'm hoping someone here might be able to help me out. Copy-pasted from there:
Background: back in 2015 I successfully installed Windows 10 on my iMac (iMac 21.5-inch mid-2011, 2.5 GHz Intel Core i5) via Bootcamp, using a Windows 10 license (burned onto a CD) I got as a gift. I'm pretty sure I didn't use a bootable flash drive, but as of now that would be necessary as I've replaced the optical drive with a SSD. I had some minor and common issues at first (such as having no audio), but eventually those were resolved and the partition worked perfectly. I no longer have this partition and am interested in installing the OS again, but I don't remember how I did it back then.
Issue 1: Boot Camp Assistant claims 'Windows 10 is not supported on this Mac'. Support software is already installed onto a flash drive. How would I go about installing Windows 10 if it's unsupported, and how did I manage to do it in the first place all those years ago? Did something change between then and now? I have tried some tricks that supposedly allow you to install the OS on an unsupported machine but those ultimately failed.
Issue 2: A little while ago I tried using Parallels and it ran rather well, except that I couldn't use my Windows license key because it didn't register my computer as being the same one where I previously installed Windows. If I manage to install the OS again, would it accept my license key again?
Any help is appreciated.
UPDATE 4: I managed to revert my shameful and convoluted partitioning mistake by wiping out disk0 and reinstalling macOS + restoring a backup. Once everything was back the way it was, I used Boot Camp Assistant to create a partition for Windows (BOOTCAMP). A new problem arose, however, when the computer rebooted and all I got was a black screen with 'No bootable device found'.
Upon pressing Option while restarting, I understand that there should be a legacy Windows option and an EFI Boot option. Because my iMac no longer has an optical drive and I am using a bootable USB, only the EFI option appears. I now believe this is the source of most problems I have come across. Something about the disk becoming a hybrid MBR-GPT and Windows (EFI Boot) not recognising it as GPT? I will look more into this and update.