Bootcamp how to install windows and keep mac os?

Question

UEFI

Guys, please tell me I licked a lot of topics, but unfortunately I didn’t find anything suitable. The point is this.
For my needs, I'm trying to install windows 8. 1, as a second operating system on imac 2011.
I changed the bootcamp config file to write the windows distribution to a USB flash drive. After recording and creating a partition on the hard drive, I reboot and then a black window appears with the inscription:

No bootable device --insert boot disk and press any key

(Unfortunately, I have not yet tried the boot from disk option, I don’t have it at hand).
Also tried loading via the option (alt) key. and the choice of booting from a USB flash drive. There, the download really went, but the matter did not move beyond the choice of the installation section. the bottom line is that windows writes:

This disk cannot be installed. The selected disk contains an MBR partition table. On EFI systems, it can only be installed on a GPT disk

Unfortunately, formatting, deleting - and creating a new partition did not give any results, I also tried to install a certain refit utility and boot from it - there are no results either.
Please tell us in detail how to install windows on imac, provided that windows must be a third-party OS, and the main Mac os. And at the same time without losing information on mac partitions?
Hypervisor options are not suitable

Answer 1

I had problems installing Windows not on an iMac, but on a MacBook Pro 2011. Through normal boot from any device, the installation did not start, the installation did not start from a flash drive, some device, it seems that the internal CD / DVD drive did not work , and the system did not see an external plug-in just to start the installation.
In the end, I solved the problem by installing Windows using Winclone.

Answer 2

geektimes.ru/post/148398 - a good instruction. Helped me install Windows on Mac Pro 1.1 using Winclone.